Problem:
	I can not login to webmail, although all info have been entered correctly. POP3 account can still be accessed through Outlook. It happen with ALL hosts and email accounts. Error while login: "Webmail's login page keeps loading" Error logs in /var/log/psa-horde/psa-horde.log : Nothing

Please read this article first: http://www.vinacis.net/content/view/28/32/

If it doesn't help it can be a conpflict with Zend Optimizer. Yes, we are not absolute sure but some server in VinaCIS Data Center have exactly this problem when running PHP 5.1.2 with Zend Optimizer 3.0.1

Please try to switch to Zend Optimizer to 3.0.2 (version 3.2.8 has also problem with PHP 5.1.2, so we recommend Zend Optimizer 3.0.2)

Restart apache and see if it works.