AirPlay with AirPort Express 802.11n 2nd Gen. (A1293) does not work with older devices

Since I exchanged my AirPort Express 801.11n 1st gen (A1264) with 2nd gen (A1293) devices, I can't stream music anymore with older devices (e.g. iPod Touch 4 with iOS 6, iPad 1 with iOS 5.1.1, iTunes Mac 11.4). It does work though with an iPad mini 4 with iOS 12.3.1 and an iPhone SE running iOS 10.3.3.

I first thought it may be related to the firmware update 7.8.1 which added Airplay 2 support to the AEX but also a downgrade to 7.6.9 (or even older) did not help. The AEXs show up as targets but iTunes shows an "unknown error (10)" while the iOS devices just switch back to local playback.

Using Airfoil, I am able to stream audio using firmware 7.6.9 but with 7.8.1 I get a "403 access forbidden" error.

According to Apple's Airplay support page, there are no minimum iOS or iTunes version requirements for Airplay 1 (for Airplay 2, yes) so I'm a bit lost to see what the issue is.

I already tried resetting the AirPorts to factory settings, creating new plain vanilla configurations, etc. but to no avail.

If I plug in the 1st gen AEX, everything works but these are running much hotter and their power supply tends to fail over time...

Any hints are much appreciated :)