There is a very common  issue where Horde prevents access to webmail by repeatedly redirecting viewers to Language Selection screen. The following commands, executed as root, will solve these issues:

root@server[~]#/usr/local/cpanel/bin/checkperlmodules
root@server[~]#/scripts/fullhordereset
root@server[~]#myisamchk -r /var/lib/mysql/horde/horde_sessionhandler.MYI

If still you have problem then check the ownership for session directory present in/var/cpanel/userhomes/cpanelhorde directory.

root@server[~]#ll /var/cpanel/userhomes/cpanelhorde

drwx–x–x 4 cpanelhorde cpanelhorde 4096 Jun  5  2009 ./
drwx–x–x 7 root        root        4096 Dec 17 01:48 ../
drwxr-x— 2 cpanelhorde cpanelhorde 4096 Jun  5  2009 mail/
drwx—— 2 cpanelhorde cpanelhorde 4096 Jan 27 14:01 sessions/
Session directory should be cpanelhorde.cpanelhorde ownership recursively