Whenever I get that dialogue box you speak of, there's an error message in
it. It says things like "object is required" or "the last action was
canceled by user" or "this method not supported by object", that sort of
thing. You sound reasonably experienced, though, and you wouldn't have told
me an error message pops up without bothering to read the error message.
Tell me you wouldn't have, please.

But then, if there's no error message then you would have seen a completely
blank box with just the three buttons on it; and I've never seen one of
those. Neither have you, perhaps, and you would have mentioned it as
surpassingly unusual - but you didn't. So I don't know what to think now.
What does the dialogue box SAY?

As for the program...well, I suppose MapPath can be counted on to end in a
backslash, right? If MatPath is set by logic rather than being a constant,
and if that logic is capable of omitting the final "\" unexpectedly, it could
result in an intermittent error. But in that case you would've seen an ERROR
MESSAGE, which you don't mention and therefore could not have happened, right?