Hey guys,
mod_bw didn't make the crashes.
We tried some things and the server didnt crash for 2 days. Now we switch back settings one by one to see in the morning if server is working. Now we have the second day without crash.
I guess, at this time, that the solution is the following:
We changed "apache reload" to "apache restart" in the log rotate config files.
I hope this was the error. Seems some how to be a bug in debian 8 / apache 2.4