I can reproduce the issue with downloading a firefox installer from ftp dot mozilla dot org (via https). Or to download a linux ubunti iso :) This is how the flow looks like on the router:...

@Clive_A Is it normal behaviour that the ER8411 is sending broadcast messages continously like this? Source MAC is 00:14:78:00:00:ff / 00:14:78:00:00:aa but there is no such device connected to the...

@Clive_A Thanks for your feedback. As stated earlier I replaced the ER8411 with an old 1043ND and seems all issues went away. There is nothing much in the environment as I disconnected everything....

@MR.S Thanks, really appreciate your efforts guys. Despite I see no error when connecting to the ISP modem directly with my laptop, I'll ask them if they can see any issue on their side while I'm...

@GRL I put a switch in between the modem and wan port -> results are the same :-( I tried to play with flow control but also did not help.

@GRL No IPS/IDS enabled, firewall yes, but I did a factory reset and with default settings doing the same beaviour.