There is something wrong with the installation of Tvheadend on Linux because many people have similar problems, including me. After installation and entering the correct password, you cannot log in via a web browser.
As far as I remember, it's OK now, I reset tvheadend and ran it from root with the option: --noacl, i.e. : Disable all access control checks.
And setting everything again, i.e. login, password, etc. ... and it helped, then I could log in
https://tvheadend.org/d/8311-just-another-403-forbidden-issue