[2022] Game crashes on change of year to 1230 - Error: Failed to fetch a valid faith 'mozarabic_church'

The Geek Cupboard said: have a save game that started on version 1.5 but I then continued playing on version 1.6.

That will be the problem I'm afraid - those two patches are not fully save game compatible. The intent was you revert to 1.5.1.1 to finish any existing campaigns.

If it's not an Ironman game you can still do that, if you still have a save that was last saved under the 1.5.x patches not the 1.6.x ones.