[AccessD] You cannot record your changes because...

I have a form with a data sheet subform. On the current event of the
subform, the values from some of the fields in the data sheet subform are
put into the unbound controls on the parent form.

This was working fine now all of a sudden the form won't open, I get a
message 

"You cannot record your changes because a value you entered violates the
settings defined for this table or list (for example, a value is less than
the minimum or greater than the maximum). Correct the error and try again.

Now, as far as I know I have no validation settings of this kind, and the
table which the subform is based on is a linked table (Excel range).

Does someone have some suggestions for what is possibly causing this?