Move Rejected

I am using GNU Chess 5.07 on Windows XP SP2. Great program, but it has
a seemingly random problem. I can't seem to figure any pattern to it,
but Sometimes, I'll get an odd error when I try to move: "Illegal
move: Rejected by first computer" or something similar to that. I
promise, this move is not illegal. For example:

1. e4 e5
2. Bc4

For some reason, the game occasionally pukes on it, saying that the
first computer rejected it. I realize that I haven't exactly given
enough info to accurately track down the problem; if there's any more
info you need, ask and I'll provide what I can. Either way, I figure
you guys should know about it.

Cheers, Tim