Hi Heiner and all,
I have now tried the legacy images with my different pis and DACs, working my way down from the newest to the oldest images. Turns out that I had to use all 3 images to get the 5 different setups working (for now). The working setups looks like this (please not that it is a bit different from the one I listed initially where I made one mistake):
1. Raspberry 1 A+, IQaudIO Pi-AMP+, EDIMAX 7811UN WLAN -> m2p_default_226_wps.img
2. Raspberry 1 A+, HifiBerry DAC+ Pro (B+ / PI 2/3), EDIMAX 7811UN WLAN -> m2p_rpi_default_244_wps_ap_expand.img
3. Raspberry 1 A+, HifiBerry Digi+, EDIMAX 7811UN WLAN -> m2p_rpi_default_244_wps_ap_expand.img
4. Raspberry Zero W, HifiBerry DAC+ Zero 1.0.1 -> m2p_stretch_rpi_v249.img
5. Raspberry Zero W, HifiBerry DAC+ Zero 1.0.1 -> m2p_stretch_rpi_v249.img
Notably, I could then upgrade the max2play version online on all installations to the newest release, 2.50. An initial test with the following setup did not conclude, here I had to switch to the Raspberry Zero W:
6. Raspberry Zero, HifiBerry DAC+ Zero 1.0.1, EDIMAX 7811UN WLAN
As for now, all systems are working for 3 days and reliably stay in the network. I am noting this since my initial venture started because some of the devices were disappearing from the network after undetermined time intervals and without any good handle how to debug. Hence I though a fresh install would solve this problem. However, it just showed the incompatibility with the images as identified now.
I hope this is helpful for anyone with similar problems. However, it is not an ideal state since I now forced to run three different versions of raspbian with two outdated images. This is increasing administration load and it is canceling out any newer patches, e.g., needed for security reasons.
