This is a symptom specifically when compiling an Excel VBA project. The following error occurs:
User-defined type not defin
Late Binding
This error can occur due to a missing reference. For example when changing from early binding to late binding, by eliminating the reference, some code may remain that references data types specific the the dropped reference.
Try including the reference to see if the problem disappears.
Maybe the error is not a compiler error but a linker error, so the specific line is unknown. Shame on Microsoft!