You didn't specifically state what kind of crash do you get, did you?
acknex has stopped working, script crash in ..., ? Have you used sys_marker already?

It could just be an action (a function) from a previous level that continues to run after the level_load, which then deals with an invalid pointer (not necessarily an entity). The error would be to look for in the level from which you switch to the seemingly crashing level.
I was recently experiencing similar problems with entity functions not being stopped on level_change. Do you have let's say a rocket that shoots high into the sky or some random bird flying high? If they leave the level(_ent)/ ABT boundaries this could lead to problems later on.

I just get a "acknex stopped working" windows error.

Nothing logged out.

If it is a running script shouldn't I see it in the list of running functions? And why does it not crash when I load another level then? And the same question vice versa. If it is a problem with a function of the new level, why does it not crash when it gets loaded from another level ... really strange.

Why does it not crash anymore when I delete a random entity in the level?

I don't know, sorry. Just trying to poke you a little to get you on the right track.

I've once had "acknex has stopped working" errors after level_load in a multiplayer skateboard game. The reason was a faulty texture in a very old WAD file.

How did you track that down? Just try and error?

My first thought was just to modify the level that the crash disappears, but I fear that it would just lead into another sporadic crash in another situation as it doesn't fix the issue itself.

Is there any chance to get an external debugger to work? Unfortunately the SED debugger can't help.

I would advice you to create a blank new level, instead of the one that causes the crash after loading (just save this one somewhere, so it doesn't get changed/damaged). Then start putting everything from the older map into it one by one (yeah, it will take hell of the time, but probably will help you to track the crash down if it's related to something on that level).

Best regards!

Hm... I think I - after jcl told me that the reason very likely could be a faulty texture - just took the level, deleted maybe half of it and tried again. No error -> deleted less and tried again, until I had it isolated (it was the r25.wad if I remember correctly, should anyone else be using that).

When you have let's say such a faulty texture loading that may not cause a crash, but some other object that gets created later could write into invalid memory areas because of that - at least to my knowledge.

had such an issue once , turned out it was something todo with regions (i cannot remember specifically ) , if you use them , try deleting them .

We don't use regions.

Is there any way to identify a faulty texture properly (I don't mean just by replacing them one by one, but something like open it in Photoshop and get an error message)?

Keep in mind that the order of the functions matter for pointers and other calls, so deleting a random entity changes the order of the running functions, so it could be related to a pointer. I'm not saying it IS, only that it COULD BE
Also, another level with the same functions could be starting them in a different order, and again, bypassing this pointer problem...

The question is: How can I track this down?

Unfortunately the engine gives no hint on what it is currently processing. Until now I was able to add several log messages into suspicious scripts to identify the bugs and fix them. Now it seems the problem could be everywhere: in the code, in the models, textures ...