Support answered my email with diagnostics file - they stated that there might be problems with either my network or my wifi-card (which is a WLE900VX borrowed from one of my TO’s).
And it hit me again - this time after ca. 24 days of uptime 
So now I am switching to plain ath10k stuff - will report again, if it is still not stable…
@Taz: I cannot seem to find ath10k-firmware-qca988x-ct-htt - where did you download it?