I use TVheadend on a Raspberry Pi but also fancied trying to use on my router just to test and hit the same wall and couldn't workout what the login is
yeah... something like that... fair few fixes happened on master so without actually testing on 21.02.1, laymans guess would be to wait for fixes to trickle down...
(no idea if force/manual install of master binary will cut mustard probably not... but definately mv /etc/tvheadend /etc/tvheadend.old if you try it)