Found it...
For another application, i needed a web pdf creator. And for this, i need to enable magic-quotes.
So in /etc/php5/apache2/php.ini, i had enabled magic-quotes-runtime.
Not sure why the admin-page of linuxmce can't live with it, but this causes the problem.
I'll see if i can find a reason for this behavoir, but for now, this problem can be 'closed'...
Sorry to bother you.....