positive Update: today I wanted to continue with this problem, but when I logged Into XOA today, Master status was Green / enabled. Since no one execpt me has credentials to the Testcluster, I am sure that no one else had done something. So the master host seemed to recovered itself from grey to green within the last 8 Hours. There were at least 1-2 hours yesterday between execution of "pool-emergency-transition-to-master" and still beeing grey dot / Halted. xe pool-emergency-reset-master master-address=... then added the remaining hosts to the new master.