The wiki page states that the Alliiertes Vorbehaltsrecht ended when Germany was reunited in 1990. The only sentence indicating anything else seems to be a reference to some "secret contracts seen by Josef Foschepoth", but with no further details.
This is the same author that is being interviewed in your second link, so it sounds a bit like this is just one guy trying to sell books...
The Wikipedia article I linked explicitly says it didn't end when Germany was reunited in 1990 ("Sie gelten weiterhin.", last sentence). But you are right in that all sources originate from the historian Josef Foschepoth. If it is one guy trying to sell books or one guy speaking an inconvenient truth, I don't know. If someone can point to an independent source I would be grateful.
There's a big debate whether those secret treaties exist and if they do, whether they say what Foschepoth says they do - i.e. if they allow spying on all german citizens by allies secret services - and if they allow it, whether the german government can sign (secret) treaties that violate principles of the german Grundgesetz (constitution). See for example this blog post http://www.internet-law.de/2013/10/darf-die-nsa-in-deutschla... and the following discussion.
So if the lawsuit ever makes it to court, these questions need to be debated - and I'm very interested in seeing the result.
Edit: Just for clarity: The situation may be quite well that currently the german governments allows allies surveillance based on contracts that are illegal in itself.
The wiki page states that the Alliiertes Vorbehaltsrecht ended when Germany was reunited in 1990. The only sentence indicating anything else seems to be a reference to some "secret contracts seen by Josef Foschepoth", but with no further details.
This is the same author that is being interviewed in your second link, so it sounds a bit like this is just one guy trying to sell books...