Recover 'unreachable'

fdbcli --exec status:

Using cluster file `/etc/foundationdb/fdb.cluster'.

Could not communicate with all of the coordination servers.
  The database will remain operational as long as we
  can connect to a quorum of servers, however the fault
  tolerance of the system is reduced as long as the
  servers remain disconnected.  (unreachable)  (reachable)  (reachable)

  Redundancy mode        - double
  Storage engine         - ssd-2
  Coordinators           - 3

Question: How to make the unreachable shard reachable again?
Everything looks fine - but the shard still remains unreachable.

Is the process up and running? Without more information (like logs etc) it is hard to know what is going wrong.

One thing you can try is to move the coordinators to a different set of processes (using the coordinator command in fdbcli).

Yes, up and running.
I saw tons of GB “moving data” from the other two machines.

But 4501 remains unreachable.
Is it necessary to shutdown/restart the cluster?

FDB: 6.0

Stop. A simple systemctl restart foundationdb fixed everything.
But thanks a lot lot your answer!