[SOLVED] OVMAPI_6091E Discover cluster failed. Worker server not found.

I tried to upgrade my OVM server form 3.4.1 to 3.4.2.

I got this error message in my OVM Manager screen:
 OVMAPI_6091E Discover cluster failed. Worker server not found.

I solved it by placing my OVM in mainanance mode myself.
I got this message in my test environment where there is only one VM server in the cluster.

[SOLVED] Cannot Rediscover Server OVM Server

Job Internal Error (Operation)com.oracle.ovm.mgr.api.exception.FailedOperationException: OVMAPI_4010E Attempt to send command: discover_server to server: ***.***.***.*** failed. OVMAPI_4004E Server Failed Command: discover_server , Status: org.apache.xmlrpc.XmlRpcException: exceptions.TypeError:rpmdb open failed

FIX:
login as root on the ovm server
rm -rf /var/lib/rpm/__db*
rpm --rebuilddb

Rediscover Server in OVM Manager