Error Message "Move = violation of chess rules"


Good evening everybody,


I am playing chess on iGoogle, powered by Today there was an error message while I wanted to move my black rock from field a8 to a2. Please notice the picture in the appendix!


The error message wanted to make me believe, that my move from a8 to a2 was a violation of chess rules, and that I should make another move. I tried several times to move my rock from a8 to a2, but there was this error message again and again and my move was not accepted by the program.


Is there a chess rule that I don’t know? Or is it a program-error? Perhaps a program-error due to 3 black rocks in this particular game?


Thanks for your answers! Heinrich (Chess-Teacher)