I had the same problem, couldn't authenticate using pop3 or imap and didn't get any error message to help solving the problem.
Here's my solution:
For some reason imap.so was missing from my php.ini (or extensions.ini as I'm using FreeBSD). The imap -php-module is needed for pop3 which I didn't know until I opened the /auth/pop3/lib.php and saw the imap_open() being used there. Then I tried to get something to show up in the logs by creating a sample pop3 login php-script, and running it from the command line. And then I got an error about missing imap.so module (it was there, but after php upgrade it was missing from the php.ini so it was not being loaded).
FreeBSD has mail/php4-imap in the ports collection
Debian does it with apt-get install php4-imap
And finally if compiling manually from source, remember to build imap first and then configure php with imap using the --with-imap=/path-to-imap-dir
Hope you find this useful (and also that this topic wasn't already covered somewhere).
Hardware and performance
This discussion has been locked because a year has elapsed since the last post. Please start a new discussion topic.