Same exact problem here. Started in the last week. Im on Chrome Version 66.0.3359.139 (Official Build) (64-bit) on MacOS 10.12.6.
But clearly it isn't limited to us and others must being having this issue as well which means its on Comcast/Zimbras end.
FIX IT PLEASE!
Thank you for bringing this to our attention. We found an issue of one of the server redirects affecting a small subset of our customers, and unfortunately it seems you were part of that set. Our apologies for that.
We have corrected the issue and put in some alerts for the future so it shouldn't happen again.
This should now be resolved for you. If you are still having issues, please log out, clear cache and cookies, and try to log in again.