When a library has the ChildNeedsGuarantor system preference enabled but has child patrons in their database without guarantors, those patrons cannot be edited. That can cause various background processes to fail in non-obvious fashions. We give a message on the System Information page if there are patrons with unauthorized borrower relationship values. It could be helpful to do the same with guarantorless children ("child" here meaning "any patron in a category that allows guarantors").
+1 - checking in an item that was charged to a child account without guarantor and ChildNeedsGuarantor pref on implies that there is a useful error message to be seen on the About Koha page, and it's... not.
Other guarantor issues cause the same problem and the same unhelpful output. I have seen this when trying to check in a book from a child who has another child as a guarantor. Also a child who is reciprocal guarantor with another patron. It's so confusing for libraries and support teams.