IODispatcher Could not add merge job to queue: Queue full

After few hours of continuous crawl speed decreasing, the whole instance becomes unresponsive, and the log is full only of this endless message repetition:

W 2021/05/19 17:34:31 IODispatcher Could not add merge job to queue: Queue full
I 2021/05/19 17:34:31 HeapReader close HeapFile citation.index.20210517145736376.blob; trace: net.yacy.kelondro.blob.HeapModifier.close(HeapModifier.java:82) -> net.yacy.kelondro.blob.ArrayStack.unmountBLOB(ArrayStack.java:276) -> net.yacy.kelondro.blob.ArrayStack.unmountBestMatch(ArrayStack.java:323) -> net.yacy.kelondro.rwi.ReferenceContainerArray.shrinkBestSmallFiles(ReferenceContainerArray.java:403) -> net.yacy.kelondro.rwi.IndexCell.shrink(IndexCell.java:193) -> net.yacy.kelondro.rwi.IndexCell.access$900(IndexCell.java:65) -> net.yacy.kelondro.rwi.IndexCell$FlushThread.flushBuffer(IndexCell.java:174) -> net.yacy.kelondro.rwi.IndexCell$FlushThread.run(IndexCell.java:123)
I 2021/05/19 17:34:31 HeapReader close HeapFile citation.index.20210517145715984.blob; trace: net.yacy.kelondro.blob.HeapModifier.close(HeapModifier.java:82) -> net.yacy.kelondro.blob.ArrayStack.unmountBLOB(ArrayStack.java:276) -> net.yacy.kelondro.blob.ArrayStack.unmountBestMatch(ArrayStack.java:324) -> net.yacy.kelondro.rwi.ReferenceContainerArray.shrinkBestSmallFiles(ReferenceContainerArray.java:403) -> net.yacy.kelondro.rwi.IndexCell.shrink(IndexCell.java:193) -> net.yacy.kelondro.rwi.IndexCell.access$900(IndexCell.java:65) -> net.yacy.kelondro.rwi.IndexCell$FlushThread.flushBuffer(IndexCell.java:174) -> net.yacy.kelondro.rwi.IndexCell$FlushThread.run(IndexCell.java:123)
I 2021/05/19 17:34:31 RICELL-shrink1 unmountBestMatch(2.0, 67108864)
W 2021/05/19 17:34:31 IODispatcher Could not add merge job to queue: Queue full