Q:

German tv-seriers about paranoid man believing he is not in the real world

This is a tv-series made in the 70's in West Germany.
The plot is a man that senses that everything is not as it seems, he feels like everything is a computer simulation. There are people after him trying to prevent him from discovering the truth. A woman is helping him.
Spoiler tags might not be needed but the ending is:

 It turns out he is right. His mind has been projected into a simulation while
 he is beeing cured of a disease. They try to prevent him of discovering the
 truth to prevent his mind from harming itself.

A:

This sounds very much like Welt am Draht (World on a Wire) (1973), directed by Rainer Werner Fassbinder and starring Klaus Löwitsch in the lead role.  It is based on the novel Simulacron-3 (1964) by Daniel F. Galouye.  The same book was filmed again as The Thirteenth Floor (1999).
Welt am Draht was however not a series, but a two part made-for-TV film, at least in the version I saw. IMDB agrees with this.

