Have you tried resetting the password hash? i haven't tested it in 8 but a good write-up on it is available here: https://www.hurricanelabs.com/splunk-tutorials/splunk-7-1-performing-a-splunk-password-reset
Looks like the hyperlink added the period onto it and broke the link. I just removed the period from the link and it should work now.
The easiest way is to just to reset it by adding a
user-seed.conf, deleting the existing entry in
$SPLUNK_HOME/etc/passwd, then restarting splunk: