Jimmyb strange. I has the same message, 'could not connect to vpn'. I ended up contacting Proton and they gave me a bunch of stuff to try including the following:
Clear the app cache in settings
Sign out of Proton VPN and sign back in
Disconnect from the VPN and forget it in settings and try re - adding it
Uninstall the app and reinstall it.
Don't know if you've tried any of this, but I hope you can get it working again. I just ended up staying signed out for a few days and it worked normally after. Sorry that's all I can come up with for now. :)