Makrus When trying to add to the calendar, do you get prompted to pick an app? I just tried with Google Calendar (don't have my personal Proton stuff set up in my testing profile) and it works. The system calendar app also crashes on my phone since it doesn't seem to have the required activity, which I think is expected. If it's picking the system calendar by default, you can try to clear that default app preference.
Try going to the system calendar app's app info page:
Settings > Apps > All apps > *3 dots in upper right-hand corner* > Show system > Calendar > Open by default
From there, do you see a button that says "Clear default preferences"? If you do, try tapping that, then try adding the event to your calendar again, but this time make sure you select Proton Calendar.