Unable to connect to Project Online from Project Professional

I just got a new ThinkPad X1 computer, and I use Xfinity as my home Internet provider. When I attempt to connect to Project Online from Microsoft Project Professional, I receive the following error message:

• Project was unable to establish a connection with [my URL]. This could be caused by a loss of network connectivity or inadequate permissions to connect to Project Web App.

Here are few additional important details:

• This problem does not occur with any other computer trying to connect to Project Online with the same account on my home network.
• This problem does not occur connecting through other non-Xfinity networks. I can reproduce this error at my home, and also at my friend's house who is also using Xfinity.
• When I connect to my iPhone Personal Hotspot from my computer, this problem does not occur.
• I installed VPN software on my computer and connected to the VPN on the same Xfinity network. The problem does not occur while connected to the VPN.
• This is a highly reproducible problem -- happens every time.

I believe this is an Xfinity issue; perhaps they are blocking me from connecting to certain Microsoft sites. I saw on this forum that others were having a similar issue recently connecting to Microsoft sites, but they didn't mention Project Online specifically.

I need to use Project Online every day for work, so this is a very urgent problem for me. Can anyone make any suggestions?

Thanks!