ProtonVPN gives an unexpected error when I connect on Debian 12. What should I check?

Since the app opens and the error happens when you connect, the installation step alone doesn't tell us where it failed. Check whether the guide has any desktop environment or dependency requirements, but I wouldn't assume GNOME is the cause yet.

Try another server and note whether it fails the same way. Then look for an app log or diagnostic option and check the system journal around the time of the attempt. The underlying error may be more useful than the message in the app. Remove account details and anything else sensitive before sharing logs.

For troubleshooting, include your desktop environment, how you installed the app, and whether ordinary internet access works without the VPN. The app version and any relevant log message would help too. That should make it easier to narrow down whether this is a dependency, network, or connection issue.