I am having the exact same problems you are having moving from 2017 to 2020 QB Pro.  So frustrated.  Looks like you haven’t found a solution.  We have to have 2020 or we cannot sync with our bank online.

We have new computers and server, running Windows 10.  Our hardware, server are not the problems.  The problem is in QB 2020.  After I first installed it, it was OK for a short time. 

Have you found a fix for QB Pro 2020 yet?