Thank you for your efforts to reproduce what was for me a bug. I am very impressed with your taking my report seriously.
Unfortunately, it appears that this problem only occurs on my computer. It could be from any installed program that scans files, including antivirus, Dropbox, or Windows Defender. At the time, I didn't know how to find the processes that have a given file or folder locked, but I have since found that LockHunter does a very good job of doing just that (as well as breaking the locks if desired).
Since, other than this problem, I had little problem in going back and forth between the 2018 and 2020 versions of Bitnami WAMP using LockHunter if needed, I'm no longer affected by this bug.
I believe I have completed the migration of my own self-written business applications and configuration files between the two versions and all seems to work. If I have a problem in the future, I can always return to the previous version to help in fixing the problem.
As I said above, if I had known that migration problems would cost me a day or so of work, I would not have been so quick to use Bitnami WAMP Stack. I had previously installed all my WAMP components separately and did not find separate installation to take more than about half a day the first time, and even less time when it was necessary to upgrade just one component.
If I were creating a stack of components similar to this, I would most definitely include the ability to upgrade just one component just by choosing the desired version, with full back-compatibility and history capture, maintaining a public knowledge base so that component migration solutions could be communicated amongst my customers, to help those customers with less experience.
One idea for you: add to the manager-windows.exe the option to attach to the task bar such that it can be used without triggering UAC. If necessary, this can be done by simply keeping the window in existence but invisible while the user is not needing it and while the manager is not quit out of.
Best of luck.