Calculated date field causing error that disappears when field is hidden

Calculated date field causing error that disappears when field is hidden

I have a field "Art Due Date" that is calculating a date for 5 days before another date in the form.

It's been workin for 2 years and then last week every time someone tried to edit a record using the form with this calculated field they would get this error "Invalid Field Found Field Name : Art Due Date" and would be unable to save the record.

I changed the setting on the field properties by unchecking the " Make the field visible in the Form" and I was able to successfully save/revise records. 

I would guess that this is a bug in the platform.