Getting "Authentication required!" error on an F5 LTM deployed within AWS

So I have a deployed a F5 LTM BYOL image in AWS (version 11.5.4) - this has no public IP assigned, as this is being used in a lab environment to prototype internal app traffic load balancing. However, I have a web server which is effectively being used as a gateway, so it has a public EIP assigned. The web server config routes SSL traffic of this F5 to the IP address set up as the management interface, this is done via an external DNS provider which I have set up an A record to match the web server EIP. The web server (gateway) config has this entry in the servername of the virtual host definition, along with a self signed SSL cert. This approach works ok for other http services I have in the AWS lab. I have SSH'd into the F5 server and set the admin pwd via the cmd line - tmsh modify auth password admin. I can get onto the F5 mgmt UI logon page, but when I enter the admin credentials I get the following error:

Authentication required! 

This server could not verify that you are authorized to access the URL "/tmui/logmein.html". You either supplied the wrong credentials (e.g., bad password), or your browser doesn't understand how to supply the credentials required.

In case you are allowed to request the document, please check your user-id and password and try again.
Error 401

Authentication required!

This server could not verify that you are authorized to access the URL "/tmui/logmein.html". You either supplied the wrong credentials (e.g., bad password), or your browser doesn't understand how to supply the credentials required.

In case you are allowed to request the document, please check your user-id and password and try again.
Error 401

This with the correct pwd as set via the cmd line... I can't find out anything more about this error... or where to look to try and determine what is happening or maybe turn on further tracing to help investigate. Can anyone help with any suggestions??