trouble with "error: event_add failed. in cpsl."

[A. Schulze]

Hello,

unbound-1.9.1 with moderate load 1k queries/s peak

after restart everything works fine but after some ours the log is  
flooded wthis way:

2019-05-24 15:49:53.874400500 [1558705793] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.888628500 [1558705793] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.892736500 [1558705793] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.892789500 [1558705793] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.900451500 [1558705793] unbound[33794:0] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.902052500 [1558705793] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.907473500 [1558705793] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.924918500 [1558705793] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.926381500 [1558705793] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.929426500 [1558705793] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.936023500 [1558705793] unbound[33794:1] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.943565500 [1558705793] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.950292500 [1558705793] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.954694500 [1558705793] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.957886500 [1558705793] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.971253500 [1558705793] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.976480500 [1558705793] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.981105500 [1558705793] unbound[33794:0] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.986870500 [1558705793] unbound[33794:1] error:  
event_add failed. in cpsl.
2019-05-24 15:49:53.998374500 [1558705793] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:54.004480500 [1558705794] unbound[33794:7] error:  
event_add failed. in cpsl.
2019-05-24 15:49:54.007178500 [1558705794] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:54.009645500 [1558705794] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:54.023203500 [1558705794] unbound[33794:3] error:  
event_add failed. in cpsl.
2019-05-24 15:49:54.037888500 [1558705794] unbound[33794:1] error:  
event_add failed. in cpsl.
2019-05-24 15:49:54.044780500 [1558705794] unbound[33794:7] error:  
event_add failed. in cpsl.

which parameter are suggested to check / modify?

this are the none zone related configuration entries:
  aggressive-nsec: yes
  chroot: "/chroot/unbound"
  do-daemonize: no
  extended-statistics: yes
  logfile: ""
  log-replies: yes
  log-tag-queryreply: yes
  harden-referral-path: yes
  harden-glue: yes
  outgoing-tcp-mss: 1220
  pidfile: ""
  prefetch-key: yes
  rrset-roundrobin: yes
  tcp-mss: 1220
  serve-expired: yes
  serve-expired-ttl-reset: yes
  statistics-interval: 3600
  statistics-cumulative: yes
  unwanted-reply-threshold: 10000
  val-log-level: 2
  auto-trust-anchor-file: root-rfc5011
  num-threads: 8
  outgoing-range: 10000
  msg-cache-size: 1g
  msg-cache-slabs: 8
  rrset-cache-size: 1g
  rrset-cache-slabs: 8
  infra-cache-numhosts: 65536
  infra-cache-slabs: 8
  key-cache-size: 1g
  key-cache-slabs: 8
  neg-cache-size: 1g
  num-queries-per-thread: 4096
  ratelimit-slabs: 8
  ip-ratelimit-slabs: 8
  do-ip6: no
  prefetch: no

Andreas