Alfred_S
Thanks for your answer. I indeed did install Proton in the owner profile via Obtanium.
ProtonVPN version seems to vary depending on its source which probably is causing your issues:
In Github (and F-Droid) 5.14.76.0 is currently the latest version, while in Google Play Store it is 5.14.87.0.
I had a similar setup and would always run into issues. Unlike you, I was able to install it from Play Store, but then kept getting error messages in Obtainium and couldn't update it from there because Github version was always lagging behind.
Proton seems to be releasing updates on Play Store weeks before publishing them on Github.
If I remember correctly, I solved this by simply ignoring Play Store updates and waiting for the release on Github, then installing it via Obtainium.