
At least I have found out which items caused the problem and that it depends on their number inside the level (more items more reports remember? And yes this is REALLY turning me off.): the reason was every item that was placed in the "basement floors" (everything lower than the first floor of the GTKRadiant editor; you'll see this ground floor if you start up GTK and make a normal brush-> where this brush has it's downside there lies the ground floor - it's the zero value on the z axis; I just hope I made myself clear - somehow

So let me put this straight: I can place as much items (literally dozens) in rooms forming the ground floor of my map as I want there will NEVER be a problem with those (except one room I'll get to that at the end of the post). But as soon as I start putting some weapons, power-ups, ammo and so on inside the rooms underneath the ground floor these are seen as "out of level items". Interestingly I can start this map without any problems if I compile it normally, no error then - even if there are items in the basement rooms. But as soon as I make the AAS THEN the error pops up while testing the map in Q3. Deleting the .aas makes that error disappear for good; no need to recompile anything.
So it seems that - since there are no leaks in my map - it's the bspc tool splitting up the map into slightly different clusters and areas than the compiling tool and throwing out some items into the void during that process.
But what can I do now to get bspc working correctly still using the -bsp2aas cmd?
Probably the incorrect splitting is caused by curves which form almost the entire 2nd basement floor? Why "probably"? Because items on B1 are also reported as "out of level" and B1 doesn't have a single curve. On the other side there is one single room construction on the ground floor again combining 4 curves to 2 rounded hallways. On top of these lies no endcap but a normal brush since GTK couldn't create end caps and I wanted to seal off these curves completely from the void using the mentioned solid brushes.
So if anyone is still reading that and could follow my explanations during the last few minutes I'd greatly appreciate his or her advises.
PS: I MUST have the possibility to place several items on B1 and B2 and I MUST have botplay enabled so I can't just ignore neither item placement on B1 and B2 nor can I just ignore the error popping up inside Q3, because it ruins tactical gameplay; and besides that it pisses me off, did I already mention that?

[EDIT: Corrected a few spelling mistakes]
[EDIT2: If it is of any help I can post the bspc console information like Herr W. did in his thread "Strange FPS boost after restart of the fraphic engine".]