Belay loop stringing

I've had this harness for about a year and a half and have not used it very intensively, yesterday I've found the belay loop like this and it seemed strange.

Does anybody know how this could have happened and if it is still safe to use? 

That looks like a pretty well used harness; it is pretty dirty.   The red thread is a warning that it is time to replace.  inspecting gear

That's a strange way for the webbing to come apart. Wouldn't hurt to contact Edelrid about it. 

I've found that flaking the rope over your belay loop on multipitches trashes the belay loop, but on the outside of the loop not the inside like yours. Chimney/offwidth climbing also severely shortens harness life expectancy. If you're going to keep using this harness it would be a good idea to tie a backup loop with some strong cord or webbing in the meantime.

All the wear appears to be in one spot on the belay loop. Belay loops are not attached so that they move around and spread the wear. Perhaps you are using it in a way that prevents this?

The area that is damaged doesn't look like it would cause failure as its at the very end of the webbing and from the look of your tie in points the harness hasn't had much wear. I would suggest you take more care with your harness, it's filthy and if I were to hazard a guess that damage is from a snag in something sharp pulling the fibres apart. 

Dal Drajnudelwrote:

I've had this harness for about a year and a half and have not used it very intensively, yesterday I've found the belay loop like this and it seemed strange.

Does anybody know how this could have happened and if it is still safe to use? 

When in doubt throw it out. Only way to know for sure is destructive testing. But you're probably fine (no guarantee). 

Dal Drajnudelwrote:

I've had this harness for about a year and a half and have not used it very intensively, yesterday I've found the belay loop like this and it seemed strange.

Does anybody know how this could have happened and if it is still safe to use? 

That is an awfully dirty and mangled belay loop for not intensive use. Usually it goes by “see red you’re dead” more common on locking biners but I’m sure that’s the red thread you’re seeing.