Thanks Jeff,
Webmail should point to squirelmail but it's resolved by the "site" instead of "directadmin". So /webmail leads to a 404 error on the website.
PHP version is 5.3 so that shouldn't be a problem.
I'm guessing that somewhere in the configuration files I messed up an deleted the link between /webmail and squirelmail.
Best regards,
Geert