Most likely reason is one or more references have been broken. The error message has nothing to do with what is actually wrong (why Microsoft has never fixed this problem, which has been around at least since Access2000, is something I'm keen to know).
Open up your References dialog box (on the Tools menu when you are in a VBA screen) and look for references that are marked MISSING. Sometimes the fix is to uncheck those references, close the dialog, reopen the dialog and check those references again, and finally close again.
Sometimes it means you need to install something on the client computers.