Random drop outs using HifiBerry DIGI2 Pro V.2.2 / Raspberry Pi 4 B / HifiBerryOS

Hello,
I'm using a HifiBerry DIGI2 Pro V.2.2 together with a Raspberry Pi 4 B and HifiBerryOS as a Roon Bridge. This device is connected via WLAN to a FritzBox 7590 AX. Putting the device together and getting it up and running went smoothly and everything seemed to work fine. The device can be reached via a browser through the local network and was immediately recognized by Roon. HifiBerryOS indicates good signal quality.
However, it turned out that at random times (minutes to hours) the connection to the device is dropped for random time intervals (a few minutes to about 10 minutes). I say “random” because I didn’t find a correlation between the drop outs and other events possibly affecting the network.
When the connection to the HifiBerry is lost, the router still shows the device as being connected, but the device cannot be reached anymore, not through Roon nor via browser. The connection returns to normal operation after unpredictable (for me) time intervals without further invention.
All other devices (PC, Mac, Android, iOS) connected to this router (LAN or WLAN) work steadily and reliably.
Here is what I tried to diagnose possible causes for the drop outs:
- Made sure that the software (HifiBerryOS and the router OS) is updated to the latest version -> no change
- rebooted the device -> no change
- Moved the device to possibly have better WLAN signal -> no change
- Checked router settings for anything unconventional and finally reset the whole device to factory settings -> no change
- Checked power supply (and connecting cable) and replaced it by a second set -> no change
I’d be grateful for your hints on how to resolve this issue. With the current behavior the device is not useful, and I'm at a loss what else to try.