No A/V output available or random monitor selected for output

As my BM Ultrastudio Monitor 3G breakout box is constantly overheating, I had to send it in. Now I have to work with AV output of a Monitor connected via HDMI. In total I have three monitors: Two are connected via TB and one is HDMI for AV. But the Monitor for AV is constantly changing, after one startup it suddenly displays AV on a TB-Monitor, then today there is no AV Output available at all anymore. The AV-Monitor is set to HD-Resolution and its on the right side of my three-monitor-setup..