I found the answer to the issue here
https://github.com/koreader/koreader/issues/5703
"Disabling user/password auth = Works
Enabling user/pass auth with auth=auto = Error 400 (even with no user/pass specified)
Enabling user/pass auth with auth=digest = Error 400 (even with no user/pass specified)
Enabling user/pass auth with auth=basic = Works"
Unfortunately im now encountering a new issue with manually entering the ip/port address

. Guess its time to start a thread in Kindle Developer's Corner.