I tried taking my SD card out of my Raspberry Pi, and booting another Raspberry Pi with the same SD card inserted. And then it all worked flawlessly. So max2play was apparently not to blame. Probably a hardware error.
The issue was not a router issue: I upgraded max2play which upgraded perl. I did not upgrade Squeezeserver to 7.9 nightly which is apparently required for the perl version.
After upgrading to 7.9 nightly everything worked (without the need to changing port for Squeezeserver webadministration)