That is, the program is working fine for many days/weeks, and then all of sudden, for no apparent reason, when the mouse moves into that window, I get the EnterFocus error.

If by that you mean that one single instance of the program is running for weeks on end then it could perhaps be that some resource is being slowly exhausted (open files, RAM, etc.). It would be a little odd if it is only one specific action causing this error to manifest but then we haven't see your code so anything's possible.