Hi,
Not sure how to report this in the bug tracker, but.... Just upgraded from 1.9.9 to Moodle 1.9.11+ (Build: 20110223) and I have a very similar issue with the guest login.
Courses with "allow guest without key" and "auto login guests" would just return the login page with "guest" stuck in the username textbox. I had to clear the cache or shut down the browser to logon with a regular account.
I saw this thread and checked my mdl_user table. I didn't have two guest accounts, but the mnethostid feild was set to "1". I changed this to "0" and now the guest login works! ?
Is there anything else I should do to fix this problem? Will it happen the next time I upgrade?
Thanks,
David