Maybe give a user pass then log in with those. Once in the web GUI try changing the username to and change the password to either blank or .
Restart tvheadend service.
Otherwise you can stop the service and then start tvheadend manually from a terminal with...
tvheadend -C -u [user] -g [group]
That will let you in to the web gui with no user/pass. Configure the username to and the pass to blank or . I forget exactly which works there. Save that out then kill the manual started tvheadend and restart the service.
Hopefully that works for you.