Hi!
I just found out (after a while of troubleshooting) that stunnel does not handle UTF-8 BOM in the PSKsecrets file, i.e. the BOM is considered part of the identity (tested with 5.44, 5.45b4 and 5.45b6 on Windows 10 1709). It was a little tricky to figure this out, as the BOM is apparently filtered out in the logs, e.g. the server just says 'No key found for PSK identity "client1"'.
Ok, nobody says that this is supported or that you should do it, it just happens automatically when you click "Edit Configuration" in the Windows GUI and click "Save as" (out of convenience, because you will already be in the config directory then), because the sample config has a BOM and Notepad will also insert a BOM in the new file.
I don't know how much work it would be to handle the BOM correctly or at least print a warning (didn't look at the code, sorry), a warning in the manual would probably sufficient, as it could be that I'm the first one to stumble over this (at least I didn't find anything about it on the net).
Regards (and thanks for this nice piece of software) Jakob