Well, i have cleaned the cache in the forums control panel (and in my browser) and did rebuild all the content.
Also set almost every directory to chmod 0777 and almost every file to chmod 0666.
I hope the is no security issue now...
The problem with the "whos online" box is solved now.
But everything else still exist.
@Ruben: I wanted to install the 7.5.7 version into a new directory (no upgrade from 7.5.6.p2) and then import the sql database (like from 6.5 to 7.x) to that new installation.
Is this possible or shouldnt I do that like you said ?
