Found it: https://bugreports.qt.io/secure/Dashboard.jspa

@sierdzio Qt was uninstalled with the Admin and install started with my regular user ID. It completed successfully and did not ask for any Admin privileges. I presume all is good and am working on learning how to use it.
Thank you for your patience.

@JKSH Hey, thanks for the answer and sorry for the very late reply.

My path: C:\Qt\5.15.2\msvc2019\bin

The bitness of the windeploy.exe in that path is 32-bit.

It seems I misunderstood/forgot how the windeployqt is used, that I need to run it for each new release. The dll:s still are of different bitness from the application's .exe but I guess that's fine then...

It works now.

@Kevin470 Hi, I have exactly the same problem with you and i also have Trend Micro Security on my laptop. I have been searching this issue for a few days. Have you tried to close the antivirus software? Could this solve the issue?

Well, my research checks out, but my solution was still wrong.

The solution worked on my computer, but it was not right on the next computer.

Turns out, it was a case of "did you try to turn it off an on again?": I didn't restart my computer after switching the primary monitor in Windows. This is the real solution!

@JonB Thanks Jon. I will look into those suggestions. I think at this point though, the best thing for me to try, rather than thrashing around with my current project, would be for me to start a new project focused on just replicating this specific issue. I'll try and pare this down to minimal required code, just enough to reproduce the problem This will make findings clearer and easier to communicate to others. If I get to a point where I have something useful to share, I'll do so. Thank you again..Mark