Is the logs directory of your roundcube installation writable by your apache user ?
the logs usually are here, but this directory have to be writable to the user running the web server.
I'm not using apache but the logs directory is owned by the same user the server runs as and in fact the sql log that I cited was created there. The actual problem was that the server was corrupting request variables so that the request token went missing. I only found out after looking through the code and inserting my own debug statements here and there, the built-in logging mechanisms simply does not log anything in this case making it very hard to diagnose what's going on.