Problem with add item in gallery

A very incidental client of mine who tries to minimize the costs on his website now has a problem. The last time he looked at his site the following configuration worked fine:

Gallery versie = 2.2-rc-1 core 1.1.29
Joomla 1.0.12
Joomla Gallery 2 Bridge 2.0.14

The problem is that when a user logs in and wants to add items to his basket he's asked to login again. The popup message is (translated from dutch):

<start popup>
Authentication required
http://www.xxx.nl asks for a username and password. The website says: “''”
Login
Username
<end popup>

Entering the correct accountdetails gives the same message again.

The hoster has upgraded PHP since the last time the site worked correctly to
PHP 5.2.6-0.dotdeb.1

Does anyone have an idea what the solution to this problem might be or what the cause of the problem can be.
I know, I know, I've already told him that the most probable answer he'll get wil be doing upgrades

Every answer will be appreciated, thnx.