See Add-Ins in Microsoft Word.

That it only happens occasionally is unusual but not unheard of. The problems with opening the document are unusual.

Despite what "help" says, you should NOT uncheck the warning prompt. This is the equivalent to taking the batteries out of a smoke detector because it keeps buzzing. First, look for the cause of the smoke. You may want to turn off the detector while you look, but you don't leave it off.