@Ethan-TP Thanks, solved.

Back to square 1. For whatever reason my ER8411 rebooted middle of a night last week. Since then same errors as before - although nothing changed setup-wise (i.e., the working setup with disabled DPI). iPhones constantly again ask for iCloud account verification (which then does not work - only a spinning wheel is presented), Several Apps run into timeouts or claim to have no internet access, some websites are partly non-functional resp. take a long time to present content, etc. Errors do not appear if devices connect to the internet via a VPN, Apps and websites then run smoothly. All of this was not the case with FW1.2.x.

What I also notice is that each router reboot regularly ends in a "PADI time out" situation (connect via PPPoE). This never happened with FW 1.2.x. No idea if this hangs together, but it's an observation made.

Are any workarounds known?

Thanks!