Client can't execute: Connection pool shut down

All of a sudden, ppm dropped to zero and the reject queue is full of these messages from many different domains.

TEMPORARY_NETWORK_FAILURE cannot load: load error - Client can't execute: Connection pool shut down duration=0 for url https://...

The error further says:

... 64 more Caused by: java.lang.OutOfMemoryError: GC overhead limit exceeded

I have 7 nodes, all randomly stopping with this error. All set to Xmx=1600 and 4GB or more system ram. If I increase Xmx further, it completely crashes after a while.
I cannot find reliable settings for keeping yacy up and crawling. Very frustrating.