'Encounters serious error' on opening with user-defined add-in
 

I have been using an Excel add-in containing my own simple procedures for
some years. I'm not sure what i've done but now when I go to open a new Excel
workbook, Excel tells me it encountered a serious error the last time this
add-in was opened and gives me the option to not open it. This is the only
way I can get Excel started. Once started, if I then manually add a reference
to the problem add-in, then I can no longer select any cell in the blank open
worksheet, but I can access the normal menu items. If I then switch to the VB
add-in code and click inside one of the procedures in one of the modules in
the add-in and then press the "Run sub/user form" button, something "runs"
for a second or two and then if I go back to the Excel sheet, I can select a
cell and operate normally.
Any suggestions as to what might be causing the problem, or how to go about
identifying the problem?