The reason I asked the above question is that Quicken must be able to establish an outbound internet connection for it to work. So there could be a problem with that. I suggest that you check your Windows security settings. Also if you have a VPN enabled that could also be blocking the connection, so you should try turning that off.
But, before you do any of the above suggestions - try shutting down all apps as well as your computer. Then restart and only open the Quicken app (no others) and try adding the account again to see if that works.
If nothing suggested above helps, I suggest that you contact Quicken Support for some hands-on assistance.
Let me know how this all works out.
Frankx
https://community.quicken.com/discussion/7924235/cannot-open-new-account/p1
Per the info from Quicken Anja in this reply, the issue has been reported:
https://community.quicken.com/discussion/comment/20318088/#Comment_20318088