ODF/CEPH: MGR not getting updates about PG states from OSDs and Ceph reports many PGs in "Unknown" state

Issue

• Ceph status shows a huge number of unknown pgs, eventhough all the OSDs are up and in
• This is usually accompanied by issues in CephFS like '1 filesystem is degraded' and 'MDSs behind on trimming'

$ ceph -s
  cluster:
    id:     740391ec-b7dc-4d73-944a
    health: HEALTH_WARN
            1 filesystem is degraded
            1 MDSs behind on trimming
            Reduced data availability: 273 pgs inactive
  services:
    mon: 3 daemons, quorum i,k,l (age 59s)
    mgr: a(active, since 3m)
    mds: 1/1 daemons up, 1 standby
    osd: 3 osds: 3 up (since 3d), 3 in (since 4M)
  data:
    volumes: 0/1 healthy, 1 recovering
    pools:   11 pools, 273 pgs
    objects: 0 objects, 0 B
    usage:   0 B used, 0 B / 0 B avail
    pgs:     100.000% pgs unknown
             273 unknown