OK, I went through all my oblivion.ini settings, oumpff .
There I found this evil one which has done this mess: fLODMultTrees. This was set to a low value and caused my problems. Now I hope that this setting isn't touched by SL and was a residue of NPFRO. If it is a setting with which SL fiddles around please tell me how to circumvent this behaviour! Thx...

You should be able to tell by looking in the Streamline INI file. You probably want to look under StreamSight's settings.

Yep, I went through this ini 10 times now. But I will give it another try. I'm very unsure about the advanced settings...

You'll see all ini settings Streamsmooth touches under Detailed Streamsmooth Settings - and it doesn't touch fLODMultTrees.

No, it hasn't been made available anywhere and I'd not be planning to do so without permission from the proper people.

What I can do is provide quick directions on how I suppressed it, which doesn't seem to have any negative side effects. I'm going to assume you or anyone else wanting to do this has knowledge enough of the CS to do this. If someone reading this doesn't, you shouldn't ask for more detailed help because you could mess up the plugin if you're not careful.

Edit the "SLfogscript" script. The first 3 lines of it should read:

Begin Gamemode  if ( SLv.Streamsight == 0 )	return

You want them to read:

Begin Gamemode  if ( SLv.Streamsight == 0 || Player.IsInInterior == 1 )	return

Save the script, then make sure you save the plugin, and then go load the game. It should be immediately as obvious as these two screens:

http://img41.imageshack.us/img41/8187/beforezs.jpg
http://img98.imageshack.us/img98/8493/afternv.jpg

I applied your tweak, but when going into an interior cell from an exterior cell, it seems the Streamline Fog settings are somehow carried over with me. Now if I save and reload after I've entered this interior cell, then yes, the Streamline fog settings aren't kicking and your script tweak does its job.

Tried to deactivate all weather and light mods but it wasn't the issue.