First off all, thanks for the replies.
Well, for some strange reason it's connecting now when set to 20/40/80. Leaving the channel at "auto" took it about a minute to connect, I've set a channel now manually, now connecting time is still around 15 seconds, no clue why, but it works. Region was set to "Germany" all the time, although I'm not sure if it does a difference since there is no option to change that for the Intel nic. Also, VHT is active.
I'm still curious, Windows is reporting the connection speed for the first 2 seconds at 867Mbps but going down then to around 200Mbps, although the link quality seems to be high (even in WinBox). No clue if this is because of energy saving features or a different problem.
Anyway, it's connecting, the link is stable from what it seems, so I'm satisfied for the moment. Will send a big file through the network later to see if the link speed goes up or stays down.
Cheers everyone for the help, I'll report later about the link speed when sending a file through.
btw, does the MikroTik routerboard R11e-5HacT Mini PciE card work on a usual motherboard with mini pcie slot as well? Was thinking about that option too when I was buying the Intel nic.