Also I am considering, that the size of the clam overgrew what RPi can handle - are there other RPi stakers here?
That seems possible. RPi is 32 bit right? My clam client (linux command line) after it has been running a while is using about 3.4 GB of address space though not quite that much actual memory. Seems close to a 32 bit limit. I'm on 64 bit system with plenty of RAM so it doesn't matter at all in my case but it may in yours.
To add more into my story.
I configured my RPi node to maintain only one connection.
I got connected to the node with timeoffset -140, BUT the timeoffset in the getinfo is "0" and the complain "block timestamp too far in the future" is not appearing in the log anymore
The block really seems to get accepted much quicker (8s compared to 2 minutes previously)
2019-01-14 17:13:28 SetBestChain: new best=888094194c94f66b793783d1b13e11268b86a2a94f3d2c69246ea649a6130d7b height=2381492 trust=888852073952277551647 blocktrust=841655184282631 date=01/14/19 17:13:20
2019-01-14 17:13:28 ProcessBlock: ACCEPTED
I allowed staking and I'll see whats happen.