I get the same error message.
my setup: Synology NAS with emby server 4.7.2.0 (docker). TVHeadend server on Synology NAS (docker). TVHeadend plugin in emby.
My tvheadend server works without problems with kodi.
embyserver.txt
Hey, you are right! I had to change the authentication type in tvheadend to both plain and digest. Now it works. Thank you