I am not yet entirely sure what caused the issues, but until today I did not encounter them again. Either it was some Port Blocking in my switch, old routing tables in the switch, general bad luck,...

I have a setup with an ER605 V2, two EAP653 and an T1500G-10PS (hardware V1, upgraded to FW V2). All devices are running the most recent firmware. Sometimes, I cannot get a connection between devices...