The readme.txt file of coming with Rotor 0.7 mentions that a castling bug was corrected. Unfortunately, that castling bug was imaginary, but the correction caused a real castling bug. I somehow managed to mix up the test for empty square with the test for unattacked square

. At the moment I am on holidays and I don't have direct access to a Windows PC, so it may take a few weeks to correct this problem. I expect that the influence on playing strength is limited since a lot of testing is done with opening books.
Thanks to Dietmar Quast for noticing and reporting this bug!
Jan