You may need to lower your overclock on that particular GPU and/or raise the voltage (if you undervolted it) to fix the invalid shares issue. Try reverting it to stock settings for testing first of all.
All gpu´s are in stock clocks and not undervolted.
With win7 everything was working 100%
Hmm, if you're at all stock settings then you're using ~250 watt per GPU? You might not have sufficient power if that's the case since 7 x 250 is 1750 watts just for GPUs...
Hey, good point, havent even think about that, have to learn how to undervolt those
You could use MSI Afterburner to reduce the mv on the core clock as well as reducing the power limit. Unfortunately you can't undervolt the memory with that program so I am using AMD's Wattman for all my tweaking right now, but that doesn't work with R9 280 series as far as I know. Perhaps wattool would work, but I'm not sure. Afterburner will help allot even without being able to undervolt the memory.
Yes you can. Next to the core voltage there's an arrow pointing down. Click on it and you can undervolt the memory