[Talking to myself again...<s>]

I've not yet solved this, however the contents of KB article #Q258734 are encouraging. It describes the exact problem and tells me that it can occur when VFP needs more buffer space. One suggested resolution is to maximize the buffer size by setting both the SYS(3050,1) and the SYS(3050,2) functions to the value of SYS(1001).

Within the last month or 2, on the advice of someone who thought it would fix another issue that we had, I coded a routine to reduce the buffer size. Why that would suddenly start causing trouble today makes no sense whatsoever, but neither does anything else that I've come up with.

I'm off to comment out that buffer setting code and see whether it does the trick...