ok, i solved the problem..
I first had the nuke version 7.2 and used the webinstaller for 7.5;
that sounds very nooby and actually it is.. ops:

so i replaced the templates and configured all again (no problem for me at the moment)
But;
when i want to post something (in an existing topic) I'll get a fatal error:
Screenshot:

if it's not big enough:

Code:

Fatal error: Call to undefined function: opentable() in /[i]specificfolders[/i]/html/mainfile.php on line 298

Well this is my code in mainfile.php on lines 297-302:

Code:

function title($text) { OpenTable(); echo "<center><font class=\"title\">$text</font></center>"; CloseTable(); echo " "; }

edit: It's just in Forum's with special Rights.