Good Friday is a "quiet holiday" in North-Rhine Westphalia (and other areas), which involves restrictions on various kinds of entertainment: https://lexmea.de/de/gesetz/feiertg-nrw/6 So it's not that Life of Brian in particular was banned, but the activist group in question picked it intentionally for their screening to protest against the holiday.

In Germany you're not allowed to mow your lawn with a motorised mower or to recycle glass bottles on Sundays either.

"Banned" feels like a slightly clumsy word to use to describe restrictions such as these.

I think I'll make showing that on Good Friday a tradition now.