Oh, since you’re seeing some GUI I presume it was just some weird Vulkan thing. Can you launch it from a terminal to see the logs again? Does proton have a verbose logging flag or environment variable you can use?
EDIT: looks like maybe ~/.cache/protonvpn/logs contains logs too? Might be worth checking that.
EDIT 2: looks like the post that I saw showing the log location is old. Seems to be ~/.cache/Proton/VPN/logs/vpn-app.log now.