Hi Lars,
I checked delta merge related parameters, also auto merge option for biggest tables and everything is in place.
But you (somehow 
) navigated me to check also MVCC statistics and there I found root cause of this issue. By selecting M_MVCC_TABLES I found one external transaction running for almost 30 days blocking garbage collector. Unfortunately the connection id is -1 so I cannot cancel it (AFAIK). Or is there any way how to cancel this zombie session? Probably restart will be needed then.
Br, Jan