I have an application used by several hundred clients. It's been in use unmodified for several years. Built using VFP6. In the past week two clients reported that the mouse does not work when selecting drop down lists, and they have to use the keyboard. Funny co-incidence?

It happens that the application has just been rebuild with some changes using VFP9 - so I sent out a copy. The client reports the problem is still occuring.

What kind of external change [e.g. hardware changes OS change] might cause this?