I can see this in your logs ActiveRecord::RecordInvalid (Validation failed: Slug is invalid). Is there a particular category this triggers on?
Looking a bit more closely, it seems you may have a few categories without slugs. Not sure what the circumstances are there, but we’ll dig a bit deeper… 
@mbauman More specifically, you had a category slug that was only numbers (2019) which didn’t pass the validation check (likely added after the category was created). I believe adjusting that has made everything come good. 
Does this sound like it could be the same for you @Christina?