The first time the problem occurs after twenty days. The second time after one week. During a few minutes the wifi is slowly becoming more and more unresponsive. I have to reboot the fritzbox. I have four or five very active stations on the 5ghz band (a tv, a chromecast, two pixels) and about ten on the 2.4ghz (but less active).
What I'm reading here is not encouraging: IPQ40xx and ath10k_ahb a000000.wifi: Invalid VHT mcs 15 peer stats