If I apply a 64 or 128-sized SHADED texture to a block in a loaded 2nd level, everything is fine. If I use a SHADED 256x256 bmp or tga, walk through the level and come across the block, the engine will crash. Why is this?
Do I have to put some flag on or to set some variable? Had there beeh any known bug with the A6.22 version, which I still use?

hm sounds weird, actually there shouldnt be any problem
try to update your A6.22 Professional edition and see if it stills crash.

This won't be the solution to my problem. I want to stick to A6.22 for finishing my current game. Btw., I get no warning message when I build the level.

You could always try copying your gamestudio installation and updating that and building/running your level on it to see if it fixes your problem.

I see you're having a lot of problems with that particular version of gamestudio, and considering the history it has (warez), I think you're going to get a lot of suggestions to upgrade.

A temporary update indeed fixed the problem. What does this prove? Other users of A6.22 have used (or are still using) textures larger than 128x128. And, for some reason I'll keep a6.22.

And that's fine... You might want to try wiping and reinstalling your copy of gamestudio, since the update worked, maybe there's a small bug in the current installation. Like I said before, most users are reluctant to provide any help other than a suggestion to upgrade to users of the Pro A6.22 version, as there is a known (and really buggy) warez version of it floating around.

Deleating A6 and reinstalling? I did this a couple of days ago. I reinstalled Conitecs trial version of A6, executed my key, and upgraded to the (I guess non-infected) downloaded version A6.22. Maybe you're really right there's something about my installed version. Still, I'd hate to go through all the process again, installing A5 or A4 commercial from the CD I have, upgrading to my current version, slowly.

The code differences between 6.22 and the latest A6 pro, are not that huge, and would not take more than an hour or two to put right. I would strongly recommend a complete upgrade of Pro to the latest version.

You may well encounter other problems along the way, that will meet with the same response. Upgrade is the way to go