Hi Clayten,
I can see the problem you describe, either in xboard mode (gnuchess -x) or
in the default mode. The reason is that the FEN position has a syntax error.
There must be six data fields in a FEN position, but there are only five fields
in the one you are using. When GNU Chess parses that position, it expects
the en-passant field in the fourth position, but it comes across a counter.
Either a '-' or a valid en-passant square should be there.
If you replace your position
2kr4/pp3pp1/4b1p1/2R5/8/1P4P1/P1PP3r/2K5 b - 2 21
by this one:
2kr4/pp3pp1/4b1p1/2R5/8/1P4P1/P1PP3r/2K5 b - - 2 21
and save it in a file, GNU Chess will happily load the file.
Therefore, I think that GNU Chess is correct in complaining about that
FEN position. However, we should improve the error message.
If other tools accept your original position, it seems that they are relaxing
the FEN syntax, but I could not say for sure.
Please try GNU Chess with the fixed FEN position and get back to us in
case of need.
By the way, I would be very interested to see your web site using
GNU Chess.
Regards,
--Antonio Ceballos