I seem to remember a problem somewhat like this in the dim dark past. I
vaguely recall that I needed to put the code that accessed the form into the
workbook that contains the form. In my case, it would have been an XLA
macros workbook, but it could be similar.

If nothing else, it'd be good programming practice to put the code with the
form.

On the other hand, my memory is now trying to tell me that I couldn't put
the form in the XLA file at all, and needed to put it (plus probably its
activation code) into the main workbook, although the main code was in the
XLA file.

No definite information here, sorry. It was years ago, and I don't have the
workbooks now, I don't think. Just a couple more thoughts to consider.