cookies with multiple installations

Hi,

I had two 2.3.3 WordPress installations on the server of a friend for whom i built the blogs, each in a a different language. the config files were set with different table prefixes to make both installations possible on the same database. the installations shared the same root but were each located in their own folder. see http://www.alibayrak.nl

Although at first i had no trouble with this way of working, recently my friend notified me that he was no longer able to log into one of the two installations. After checking it out it appeared that one of the installations did log in as it should, but the other displayed the message that cookies need to be supported. That is pretty weird as the installations were basicly clones with only a different table prefix and a different language file. I tried both firefox and internet explorer with no success. Also now i upgraded the installation which had teh issues to 2.5 but now it only gives a blank page when i try to log in..

does anyone have any idea on what is causing this?

greets,
Fantast