E-mail not working in cPanel / Webmail broken

I was able to replicate the issue you were experiencing:
Notice: Unknown(): Login failed. (errflg=1) in Unknown on line 0
Notice: Unknown(): Login failed. (errflg=1) in Unknown on line 0
Notice: Unknown(): Login failed. (errflg=1) in Unknown on line 0
Notice: Unknown(): Too many login failures (errflg=2) in Unknown on line 0

I then proceeded to repair the horde session handler database and restarted the mail authentication daemon.

root@jackpot [~]# myisamchk -r /var/lib/mysql/horde/horde_sessionhandler.MYI
- recovering (with sort) MyISAM-table '/var/lib/mysql/horde/horde_sessionhandler.MYI'
Data records: 26
- Fixing index 1
root@jackpot [~]# service courier-authlib restart Stopping Courier authentication services: authdaemond Starting Courier authentication services: authdaemond

After that I created a test account and was able to login to horde succesfully