In the process of fixing another issue (Discover downloads), I tried the sign-out and sign-in trick. It worked for this account; however, it or some other troubleshooting step caused me to need to reactivate all accounts. This worked for all accounts except a Capital One account. When I try to activate it, it grays the screen and does not open the set up account window. Task Manager shows memory usage for Quicken, but no CPU time, disk use, or network use. It is necessary to use the Task Manager to kill the process. I tried logging out and back in. The problem remains.
This appears to be the same issue as community.quicken.com/discussion/7921629/cant-re-activate-account-downloads, which was closed without a resolution.
I am running Quicken for Windows version R51.12, build 27.1.51.12 on Windows 10 Enterprise
It is frustrating to solve one problem and creating two more.