Here is what someone else posted, and I edited for the step that (for me) was missing.

1. Login to your master Comcast account.
2. Go to account and identity. https://customer.xfinity.com/settings?role=primary
3. Scroll down to Account Details.  You should see your initials or such and underneath it "Viewer.  Click.
4. Scroll down and click on a sub account then click on change password.

I didn't know to click on "Viewer" so I ended up calling CSA, 888.565.4329.  There is a wait, in my case about 25 minutes, but they walked me through it, same as above but where I learned I needed to click on my Initials/Viewer to get to my secondary account.

Yes, to fix this, you need to go to the secondary email accounts and change permissions to "viewer". If they are set as manager, you can't change passwords. You may also have to remove auto bill payments to make the change. Then the viewer account will allow a change password and it is fixed. Need to probably do all secondary accounts as viewer, and then remember to turn auto pay back on.

Thank you! I will be trying all of the suggestions made on this forum and hopefully one will work.  

@user_6dsevw​ Thank you! Couldn't get secondary email on phone after forced password reset, couldn't log into the account on the web although it still worked in Outlook. After all the run arounds that everyone else experienced the solution was exactly what you said, change the account to "viewer". This brought up a way to change the password. Once changed, back to normal. 

Thanks. This is the way!

For me, CSA walked me through it in less than 10 minutes and it was virtually the steps provided @ducksoup, though, for me, it involved clicking on Viewer to get to my secondary account.  Given how easy it can be, don't understand why this isn't communicated to all Tech Support.  Then, if there are some it doesn't work for, they can go to CSA.  I also think there is more at play than just secondary account password resets.  I was thrown into a sign-in loop when trying to access my secondary email.  When I clicked on Account and Identity, it put me back to signing in.  I quit trying and the next day no problem.  Inasmuch as I was on my laptop, you may be correct about migration to the app though Xfinity has denied it in earlier posts.  But, then, Xfinity has not shown itself to be trustworthy or particularly reliable, IMO.

CSA has business hours. They are not available 24 hours a day like normal customer service.  

I have not had trouble with Comcast/Xfinity email in almost 30 years.

Someone at the executive level is responsible for this issue.  There should have been a mass customer service email sent out to everyone a week before,  warning that all secondary users need to have a recovery method or risk being locked out.

This is poor management in not doing that.

THAT SIMPLE STEP WOULD HAVE AVOIDED ALL OF THIS DURING THE BIGGEST TRAVEL WEEK OF THE YEAR.  

In any case CSA fixed my problem in less than an hour total time spent.

@ben345​ I have a recovery method and still got locked out!!

@Man1958​ 

You have to call the CSA phone number, Monday-Friday 9-6.

They will fix it.  It will probably take an hour to do this.  It’s the only way to get this issue resolved.

@Man1958​ I went into my primary account and was able to change the password after 10 plus days of not being able to log into a secondary email.  I didn't have to remove auto pay or do anything other than where it said " change password". It was resolved for me,  I hope you can get yours resolved soon,  I know how frustrating it was for me!

@Man1958​ I hope you have been able to resolve by calling the CSA number.  When I did, last week (Friday?) I was lucky enough to be on hold for less than 25 minutes.  Inasmuch as Xfinity hasn't "fixed" the issue yet, I'd say it's not going to happen.  One time when I called, the IVR even had a message about having to change your passwords and possibly multiple times during the holiday season!  I wonder if they've at least been able to stop forcing password changes on secondary accounts.  And I still wonder why all tech support hasn't been informed of how to help customers since it is so simple though there seems to be multiple scenarios.

man1958 is the primary account, my husband's. It's the only way I can post here is to sign in as him. Maybe their "tech help" or whatever it is can resolve it, maybe not. I've read so many different things here. And we've dealt with comcast long enough. I really don't feel like spending SO much time on the phone to maybe , maybe not really fix it. Currently I Can Sign Into Outlook and put in my comcast password!! I also have (whatever app it is possibly samsung e mail) on my phone don't have to use a password. Do you ever think of the (million?) of people who - maybe don't have a primary account to even sign into and post here experiencing the same issues and getting some "rote" answer from a person in a far away country after waiting an hour and even being lucky enough to get to a live person. O.K. I'm just ranting now :o). Have a good day folks, will it ever get fixed, shrugs, no clue, but I've been meaning to sign into my yahoo e mail and maybe just start transferring all my important e mails there.

I have signed in as the primary account. I have gone to my "manager" account from there. There is NO option to change password. Again, I read somewhere don't remember if this thread or a different one, to change my account status to the lowest one with no privileges from there. I guess that worked for someone else and the change password magically appeared. But I choose not to jump through that ridiculous hoop. The problem is exactly the same as day one.

@marlene1199​ I don't believe Xfinity has done anything.  If you look at the earlier posts, the key has always been to sign into the primary account and then change the password of the secondary account.  Supposedly because you were able to provide an email address and telephone number not associated with the primary account, this shouldn't happen again.  Why Xfinity's technical support hasn't been advised of this to help customers is beyond me.

@Patty-B​ If you have a separate email address and telephone number associated with your secondary account, it shouldn't happen again.  I set up another gmail account for that reason and I already had a second phone number through Google voice (it's free).  It has certainly taken Xfinity a long time to acknowledge the issue.  At first, you couldn't reach anyone unless you had the CSA number, which I have now noted for any issues in the future where I can't get help.

And xfinity should be honorable and just ADMIT they [Edited: "Language"] up! 

They way XFINITY did this disrupted thousands of lives with no explanation.  It is negligence.

XFINITY will try to jedi-mind trick you by saying "I understand this must be very frustrating.  I would be frustrated also.  We apologize for this inconvenience."  You can just stop that schtick because we all know that it's just a canned script being read with no emotion. 

I'm with Anon98762.  XFINITY *should* be honorable and publish a summary of 1) what they are trying to do (including an anticipated schedule), 2) what customers can expect as a result, and 3) how to efficiently get good accurate support should 1) not go as planned.  This should be sent in an email to all customers.  This email should have been sent out at least a month before whatever is going on here was attempted, with a reminder the week before.  Now that email should also include a huge apology and service credits for the disruption of service and time it has taken to resolve this.

About resetting your password:

https://forums.xfinity.com/conversations/signin-and-account-management/xfinity-password-guidance-for-account-and-information-security/6571e2e4cc3f661cfc863855

Could you please send our team a direct message. Our team can most definitely take a further look at this issue.To send a "Direct Message" ("Private") message:
Click "Sign In" if necessary
• Click the "Direct Message chat" icon
• Click the "New message" (pencil and paper) icon
• Type "Xfinity Support" in the "To:" line and select "Xfinity Support" from the drop-down list which appears. The "Xfinity Support" graphic replaces the "To:" line
• Type your message in the text area near the bottom of the window

Could you please send our team a direct message. Our team can most definitely take a further look at this issue.To send a "Direct Message" ("Private") message:
Click "Sign In" if necessary
• Click the "Direct Message chat" icon
• Click the "New message" (pencil and paper) icon
• Type "Xfinity Support" in the "To:" line and select "Xfinity Support" from the drop-down list which appears. The "Xfinity Support" graphic replaces the "To:" line
• Type your message in the text area near the bottom of the window

Don't waste your time on text chats!  I'm surprised Xfinity still has the audacity to suggest (message above yours, "2 hrs ago".  This has been an ongoing issue for a number of weeks, yet they still continue to lock customers out of their secondary accounts to force a password reset....which they know customer are unable to do.  The wait time for CSA is worth it as they'll have it resolved in less than 15 minutes.  Begs the question why Xfinity hasn't advised all technical support personnel to walk customers through the fix.  If you will scroll back, there are also step-by-step instructions that work though there are a couple of slight differences depending upon how your primary account is configured.

@user_en2tmh Reaching out to our Customer Security Assurance (CSA) Team is the best course of action when all other troubleshooting steps don't help resolve the issue. 

@user_m3gg72​ I worked with a CSA member for over an hour (after hours of other dead ends) and they still could not resolve this password reset issue. Escalated. I have never heard back, so now I'll start the whole exercise again.