After the long wait for FW 06.01.10, disillusionment arrived after it was installed.
The promised fix “Fixing a problem with devices on the network being unreachable when using an additional WLAN access point” does not appear to have been fully implemented.
The problem still exists that if there are additional WLAN APs in the network, the wired devices no longer have a connection. It also helps to briefly turn the switch to which the WLAN AP is connected off/on. The network will then work again for a few minutes to a few hours.