Apparently even today manufacturers of network devices fail to cope with technologies introduced a decade ago. If your host has ECN enabled unconditionally, D-Link switches won't let you connect to their web interface. At least the DGS-1210-16 I have here won't. With the newest firmware, too.
Now of course this is my own fault, given that I've switched the ECN mode away from the default "ECN server mode", i.e. advertise it, but don't use it for client connections, to "Use ECN". Still, given that RFC 3168 dates back to 2001, one would assume that network endpoints can deal with those bits (ECN and CWR) being set now. Especially as the RFC tells how to decline the usage of ECN when being the server. Instead you get back a TCP reset (RST=1, ACK=1). Thank you very much, D-Link.