My Proton Mail app would not connect either. I uninstalled / reinstalled it (F-Droid version) and still the same problem. After trying to connect awhile there was a troubleshoot message that appeared at the bottom of the app. After following the guidance in the troubleshooting section, the app works once again.
I know it's not the same app but it is overall the same service provider. Not sure if there is something similar that appears for Proton VPN.