Could please anyone take a look at:
http://www.classic-consoles-center.at/forum
I am having the problem that after “Last visited …….” there is a big gap of approximately one screen height until “New/Recently updated topics” appears. At least one other visitor of my forums has that issue too.
I am not sure if this is a problem with the theme itself (probably not, because I am sure many people use this theme) or if the problem appears only on my site – maybe the column for the forum is too small?
yes its your wp theme… or how they stack up floats… you could probably change a bunch of floats within the sp theme and get it working, but easist to just add a float: left to the #spMainContainer styling in your sp theme…
This goes beyond my skills I am afraid 
Do I have to add that in the default.php (respectively ccc.php in my case) or in the spHead.php?
You want ccc.php.
Scroll down from the top (not far) until you comne to…
/* —————
Main Section classes
——————*/
#spMainContainer {
and in that #spMainContainer block add the float: left; as Steve suggested.