I'm currently playing Morrowind and doing the mainquest in Tribunal, where I am using Antares alternative Main quest.

But I have encountered an odd problem, after I killed The dark brotherhood leader and want to search him, I can't. I can see the body but not click him, and I can't even find him using ori in the console, but the body is visible through walls.

It seems like a bug and I also earlier encountered a few npc's in Mournhold, which I couldn't interact with.

I don't believe that he is supposed to be unclickable, since I'm supposed to search him, so what can the problem be?

is another object in the way? sometimes that happens with activators that have a name. you can circumvent that by using tcl and moving in until their name prompts.

It doesn't seem that way, since Ori on him only shows the interior statics below him. Also his corpse is visible through the walls, so it seems that something is very wrong with him.

if you disable those statics, can you then click on him? odd

I'm not sure about the corpse, but not being able to interact with NPCs usually means some major screw-up with Dialog.

If when you ori you get the wall, then it means there's some collision detection issue. Do try tcl and approach it from another angle.

tried to exit/enter the cell again?

I have noticed that sometimes when you kill an enemy that falls and "sinks" into the ground a bit (or into one of the mudpools along Bitter Coast) you can't activate the corpse. Could that be the problem?

When II tried started Morrowind again, I could access the corpse of the Dark brotherhood leader, so now it's working. But I don't know what went wrong before (and I also encountered problem once before, where it wasn't important).

But the problem has solved itself

It happens to me too when NPC falls into walls and are half into the wall. I usually use 'TCL' and get close to them to dispose them. I think thats what happened to you.The wall gets in the way to activate him.