mac mini password

I have a Mac Mini, roughly 2007 vintage (I know, somewhat ancient and not offered as an option for the device, so I selected 2018 or later - apologies). I've barely used it for many years, mostly accessing shared files from Windows computers via network shares, and using its USB drive as a network location for a Windows computer running Plex Media server, booting it up as needed. I figured I'd retire it and save a little on electricity, and migrate files off it.

I cannot log into the computer with the password that has been unchanged for a very long time; the same password that the Windows computers use to access the network shares successfully. The login screen says I can use my Apple ID to change the password, but it refuses to accept the Apple ID username and password, which I also know is correct and could log into Apple websites and iTunes on a Windows PC. In any case, I changed the Apple ID password, after being locked out of doing that for the best part of a day, because apparently I didn't know the name of my first pet, the street I was brought up on, nor my favourite sports team.

Anyway, I'm logged into this site now using my Apple ID and the new password, so I definitely know the correct password.

The Mac Mini will not allow me to change the user password using my Apple ID. The reset password windows gives a little shudder when the Apple ID and Password are entered. There's no message as to why it fails. I've only ever had the one Apple ID, so can only assume it's the correct one.

If I log in as the guest account, the computer has internet access. I can still access the folders that have been shared, but there are others that I'd like to get to so I can retrieve files.

I've solved most of the problem by installing an app called HFSExplorer on a PC and extracting the media files off the USB drive, but would still like to be able to log into the Mac itself to make sure I have everything.

Any suggestions as to where I'm going wrong?