nfs stalls client: nfsrv_cache_session: no session

Hija,
  I have a problem with NFSv4:

The configuration:
  Server Rel. 13.1-RC2
    nfs_server_enable="YES"
    nfs_server_flags="-u -t --minthreads 2 --maxthreads 20 -h ..."
    mountd_enable="YES"
    mountd_flags="-S -p 803 -h ..."
    rpc_lockd_enable="YES"
    rpc_lockd_flags="-h ..."
    rpc_statd_enable="YES"
    rpc_statd_flags="-h ..."
    rpcbind_enable="YES"
    rpcbind_flags="-h ..."
    nfsv4_server_enable="YES"
    sysctl vfs.nfs.enable_uidtostring=1
    sysctl vfs.nfsd.enable_stringtouid=1

  Client bhyve Rel. 13.1-RELEASE on the same system
    nfs_client_enable="YES"
    nfs_access_cache="600"
    nfs_bufpackets="32"
    nfscbd_enable="YES"

  Mount-options: nfsv4,readahead=1,rw,async


Access to the share suddenly stalled. Server reports this in messages,
every second:
   nfsrv_cache_session: no session IPaddr=192.168...

Restarting nfsd and mountd didn't help, only now the client started to
also report in messages, every second:
   nfs server 192.168...:/var/sysup/mnt/tmp.6.56160: is alive again

Mounting the same share anew to a different place works fine.

The network babble is this, every second:
   NFS request xid 1678997001 212 getattr fh 0,6/2
   NFS reply xid 1678997001 reply ok 52 getattr ERROR: unk 10052

Forensics: I tried to build openoffice on that share, a couple of
   times. So there was a bit of traffic, and some things may have
   overflown.

There seems to be no way to recover, only crashing the client.