@Alexander_Halm ---- I saw you updated your original post to include a flag for the support team, so my assumption is that you are still experiencing this issue with the Play:3.
Moving forward, I checked the SONOS section of the community site to see if anyone else was reporting issue after taking the latest firmware update and I am not seeing any other related posts. My sense is that if this was a widespread issue, we would be hearing more about it as we have many users who are implementing various Sonos based endpoints in their setups.
In light of the above I would like to have you run a few tests to see if we can isolate some variables here to help determine where the issue could be stemming from. Please see below.
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Could you please test with thePlay: 3 mounted directly to the Fritzbox 6490 router and confirm if the application identifies the unit.
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Please try testing with the mentioned repeater (i.e Fritz WLAN Repeater 1750E) temporarily disabled and confirm how things respond in regard to the mentioned endpoint. It is important to point out that we have seen issues pop up with these types of devices in the past, which is why we recommend the use of a mesh network configuration by companies like Orbi and Eero. Our CTO Brian goes into further detail about the limitations of these type of products (i.e “extenders”, “repeaters”, etc) here.
-Eric