The log is /var/log/ispconfig/. But thats a sytem log, login problems are not logged there. Login errors are printed on the screen if any errors occurred. So you will not find any info about your problem there.
Your problem is most likely related to issues with php sessions. Have you installed nay session, cookie or ad blockers in your browser?
Last edited by till; 24th May 2011 at 16:03.