Hey partypeople, sorry for the late note but it happened at night while i was asleep. Looks like Synapse getting stuck all few weeks isn't enough anymore - it now went stuck again a few seconds after restart... investigating.
It seems to work again, the initial issue was another memory flood that wasn't stopped by the oomkiller, instead the Synapse server froze - i wasn't able to determinate the cause of this yet. I later improved our email-config and realized that i couldn't receive matrix.org messages. After hours of investigating, this is still the case but maybe some weird caching issue? The thing is that i was able to receive messages from other servers which i realized a good while later. ...right now i need to do other things and hope that it is really "just" some caching related issue...another thing i am not sure about is that my riot from time to time loses the connection, i am not sure if this is related to my internet, there are no errors in the serverlogs except of timeouts that may be clientside...not sure.