Thanks guys, but the datacenter has found the cause of the problems, after I tried to install a clean WB to see what would happen. The fresh installation didn't quite finish and stopped with the error:
/var/www/vhosts/ccmonline.nl/httpdocs/wb/framework/functions.php on line 799
Fatal error: require() [function.require]: Failed opening required '/var/www/vhosts/ccmonline.nl/httpdocs/wb/modules//output_filter/info.php' (include_path='.:') in /var/www/vhosts/ccmonline.nl/httpdocs/wb/framework/functions.php on line 799
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The I knew there HAD to be something wrong on the server, and I pushed the tech guys to keep looking for it. Luckily they found it, and told me: "The problem was caused by a limit on the number of open files, combined with a bug in Plesk/Apache and the log files".
All seems to be back to normal and working fine again.