The problem for incorrect login for Horde webmail OR the imap problem for Squirrelmail is often caused due to failure of imap service on the server.
We first need to try restarting the service. Some times the imap service is not restarted even after the following commands :

service xinetd restart
OR
/etc/rc.d/initd.d/xinetd restart
OR
/scripts/restartsrv_xinetd
OR
/scripts/restartsrv_inetd
OR
/scripts/imap

This means that the IMAP service is in hung state and it needs to be killed and restarted. When any of the above commands do not work, then try this :

killall -KILL cpimap

This should kill the imap on the server. Then restart the service using :
/scripts/restartsrv_imap
AND
/scripts/restartsrv_xinetd

PLEASE NOTE BE SURE TO TRY THE REGULAR COMMANDS FIRST THAT I MENTIONED BEFORE. IF THEY DO NOT WORK THEN GO FOR KILLING IMAP USING THE (killall -KILL cpimap) command.