Linux freezes after a time while running

[Conrad Kostecki]

Hi!
In order to create a dual band AP, I've bought two Compex LE900VX cards.
As the mainboard has only one PCIe slot, I also bought a Mikrotik RB14e miniPCIe->PCIe adapter, which can carry up to 4 miniPCIe cards.
 
Currently, I am running HostAPd 2.6 and Kernel 4.8.4 with 10.2.4.70.56 ath10k-firmware.
Both cards are being detected fine and working.
 
When I do start HostAPd, the whole server just freezes after a time, usally after 30-120 minutes. Until it freezes, HostAPd is working perfectly fine. When I do not start HostAPd, no freeze occurs and the whole system is running stable.
 
No errors were logged in dmesg.
 
Any Ideas, how to debug this?
Are the cards maybe faulty?
I already asked on the HostAPd-mailinglist and was asked, to ask here ;-)
 
Cheers
Conrad