Hi Aniek Ivens,
I have posted your request in another forum and I hope that there will be one or the other ant-keeper from this area (Hamburg to Rostock) who can tell where to find Lasius flavus:
http://www.antstore.net/viewtopic.php?t=6110
The species is one of the most frequent in northern Germany and you should find it there in similar places as in the Netherlands: in pastures and meadows.
It is not necessary to go into a nature preserve, which would mean that you had to apply for a special permit from local authorities to collect there.
But in Germany you may enter any private property (except for fenced gardens around homes), be it agricultural area or a forest, to look for your ants. When going to a farm it may be appropriate to ask the owner, if there is a chance to see him. Nobody will mind if you just go onto a pasture (even if fenced) and dig around a little in the ant mounds. Just beware of bulls that may be on the pasture
In the antstore forum and in other German forums the ant-keepers quite often discuss on keeping L. flavus with root aphids, but I guess that as yet nobody has really succeeded.
Lets hope that we get a reply.
Best wishes,
A. Buschinger