Sorry for last night’s downtime. Turns out trying beta versions of software isn’t a very good idea (even more so when you don’t backup first), I somehow managed to mess up the permissions (I think), and overwrite the themes and plugins, which made the site unvisitable for about 24 hours. 
Should all be working again, thanks to Minthost’s backups (and Jonathan’s help). >_<
Update (17th July 08): Sucessfully updated to Wordpress 2.6 final. 
It could take shorter though if you wouldn’t refuse a helping offer
Glad to see it’s fixed