Haven't seen this before. Which version of Access? Have you done much coding behind it or is it fairly straightforward?
The fact that two independent companies have come forward suggests it might be update related to me otherwise I would have suggested volumes but for both of them to hit a particular volume where the database had a problem or became corrupt in the same time period seems unlikely or a big coincidence.
Can't they send you a copy to take a look at and try on your machine?
Have you searched MS knowledgebase ? If a Microsoft update had this effect I think it would be in there fairly quick.