Time for a small update: I thought that maybe the error was due to the fact that I had used Smart Switch to migrate from Note9 to Note10+. So I uninstalled the secure folder completely and re-installed all its components without using any backup-/smart-switch-data. Result was that at least for 1 day the folder would start flawlessly. In the meantime, I am back to square 1 with the "freeze" problem now reappearing most of the times I am trying to start secure folder.
Only workaround is the "dedicated fingeprint" tip that
@abood99 shared. Unfortunately, the error is not limited to the case that
@juliesdroidsync describes. However, if clicking on a notification in the notification tray as described, chances are that even the workaround "dedicated fingerprint" does not work anymore. In such case, the only workaround is restart.
So all in all, the problem is more than just a nuisance. I am surprised to see that only very little can be found online on this issue. Let alone how little is done on Samsung's side to address/even fix the problem.
So sad!