Problem streaming from iMac to Airport Express after firmware update

I updated two Airport Express' with the 7.8 firmware. Since then the iMac has problems streaming from iTunes to the AEs while iOS devices and MacBooks work without any problems after the update. iTunes version: 12.8.0.150

I am getting an unknown error message (9039) when trying to connect to either AP Express on iTunes.

Reverted FW back to 7.6.9 which did not restore original streaming functionality, now on 7.8 again. Also, restarted Mac, and repaired with Disk Warrior, no difference.

Effect: I can see both AE in iTunes on iMac but with a minus sign, activating them works for two seconds (without streaming to AEs) and then reverts back to the minus sign without ever having connected to either AE while at the same time stopping playback.

Setup: iMac 27 inch, mid-2010, with OS 10.13.6 connected to Airport Extreme (FW 7.7.9) via Ethernet, AEs connected via 2.4 GHz network.

Any ideas where the problem could be? Any hints highly appreciated. Thanks!