update
In the meantime I figured out it has nothing to do with devices connected by the network.
It seems 'bluetooth' is guilty. As long as bluetooth has a normal status, nothing happens, but when I get to see 'bluetooth not available', which happens almost always when working a longer time on my macbook. There is probably something wrong with the internal bluetooth-hardware which causes this stand-by issue...
Solutions? None, restart the computer can help but most of the time, the only thing I can do is shut down my computer for some time, 5 minutes or something, maybe longer, and afterwards it's working again...
Has it got something to do with the heat? I don't know...
Yesterday I called the support center: they gave me this article number 304308 which I should give to the people of the report center. There are a few hidden documents for customers, but they can see them in their computer. It describes the failure and the solution for it. The man on the phone said: 'apple gives in some cases a warranty exception on this failure, so when your mac is already out of warranty, you can still have your mac repaired for free'...
Next week I'm planning to turn my mac in if I can't find out a solution myself the next couple of days... I'll submit an update afterwards