User Form - Exception Occurred

I have a user form with 48 checkboxes on it. To my knowledge I've not made any changes to it and it was working ok yesterday but now when I click some of the checkboxes it causes an 'Exception Occurred' error to pop up. To try and troubleshoot it I created a copy of the workbook and deleted all the code from the user form but still when those checkboxes are clicked the error occurs. If I delete the offending check box and put a new one in it's place it's OK.

Any idea whats happening here? I could just delete all the offending check boxes and put replacements in there but I'm worried the problem might surface again.