are you able to access the Chat URL directly ? http://<midtier url>/eschat/chat.jsp#/chat
I had resolved similar issue by verification my authentication configuration. Setting an authentication method - Documentation for Live Chat 19.08 - BMC Documentation
This is how my authentication configuration looks like:
side note: I still have some other issue where users unable to recieve agents chats or vice versa.
Yes I am able to access chat url and I have the same configuration for authentication, for rsso agent file it's there also.
In your environment what is the eschat_url configured in the centralized configuration in smart it section ?