Screen sharing on High Sierra—why isn't it working anymore?

Screen sharing has stopped working on my 84-yo mother's Mac Mini from 2011 running High Sierra. I keep getting this error regardless of whether I connect from Macs running High Sierra, Catalina or Monterey:

The firewall isn't blocking: screen sharing is mentioned as permitting incoming connections, but even if I turn the firewall off screen sharing won't work.

I've tried to add my own account under Users & Groups and added that in Sys Prefs—Screen Sharing but it doesn't help. It also doesn't work if I allow everyone to connect.

I have also tried to share using Messages app on my Macs but I get the same error.

Can someone help me with this? I am having a hard time helping my mum since I live in another location and she quite frequently needs assistance with the computer.

Thank you in advance

Philip