Oracle VM Server: OVMAPI_2005E and OVMEVT_003503D_000

In my OVM environment i was trying to delete a server from a pool and this did not work. The error i got signalled was:

OVMAPI_2005E "[ServerDbImpl] xx:yy:zz:... (oraclevm)" contains a component "xx:yy:zz:..." in error. Error event: server.cluster.failure., Summary: Server Cluster Failure, Description: Failed to leave cluster OVMEVT_003503D_000 Server reboot is required..

I rebootet the server: still the same error.
I rebootet the ovm manager: still the same error.

Then i realized, that there was a red cross at my server:

The solution was: Select events from the drop down for this server:

There i had to "Acknowledge" the events and after that i was able to remove the server without any further error...