I attached a game I saved before and after the computer made an illegal move.
The Black Pawn G4 takes the White Pawn F4 - in the F3 Slot that is Empty.
The Move History clearly shows the illegal move.
I loaded the game after saving it, which gave me the error No position has been loaded yet
The debug info:
$ xboard -debug
GameListBuild 0 msec
xboard: No position has been loaded yet
GameListBuild 1 msec
GameListPrepare 0 msec
That's all I can give you, hope that helps.
Jeff Flesher