That date format isn't "invalid" in the strict sense. Instead, that date format is rejected by the local template. That's why there was a red error message in the context menu, when the parsed template was displayed:

The only reason it generates an error message is because the local community wrote the template to actively discourage the use of that date format. Only some wikis have a local rule against using that date format, and only some wikis have templates that use error messages in some citations templates to discourage its use. (And more wikis don't have the rule, but do have some templates copied from a wiki that does have that rule, so they get the error message even though the format is not an error on their wiki.)

I'm not convinced that it makes sense to make the software enforce stylistic rules that only exist at a fraction of wikis.

Yeah, I agree that we shouldn't enforce date styling conventions in code, but we should certainly try to get that site working in Citoid. :-)

Works now: I suspect this was caused by T98782.