If anyone is following this: in the Slack community it was mentioned that this is a known bug, to be fixed in 9.13 (ETA end of April).
If you’re using content that can be edited in your app by a widget like Rich Text Editor or CKEditor, this error may be caused by users pasting formatted text from Word or something like that. We’ve seen something similar and encourage users to use ctrl+shift+v (paste as plain text) and also detect on change if generating a pdf is possible and if not, reset the edited content.