After some long time testing I've concluded that the USB contoller on my NUC6i3 was the problem. I connected the enclosures to my HTPC and I had no problems with the pool the next 30 days. Bummer... :-( , but also a relief to finally know why I had such problems.
I'd like to say thanks to Christoffer for giving me some pointers in finding the cause of my problems.
I have now made the decision of turning my 5-year-old high end gaming pc into a VM-/file-server. I bought some extra RAM, and a LSI SAS-card to get enough ports and speed for my drives.
Now I have to get myself familiar with Hyper-V and how to set up VMs. A lot to learn, but I'm enjoying the entire process.