I was away for a while, but posting just to close the topic, if anyone would have similar problems.
The issue was (as expected) that the video output (monitor) was connected to one of the 4GB GPUs.
Since this rig has 8 x 4GB cards, I have connected the monitor to the motherboard and have switched to HiveOS.
This has worked for short time, but then I suppose the DAG file became too big. Due to some time away, I have turned off the rig.
Now that I am back, does anyone know if plugging 8GB card in the 1st slot will keep the 4GB cards mining ETH in the future, I suppose not?
The first slot is the most important and I believe that's where you should connect the display cord too, the motherboard needs the first slot video ram to process the whole mining, that's how many got away with 4GB GPUs and 1 8gb GPU, I'm sure it's still working for many, try it out and drop your result, good luck