Hello,
To understand the root cause of the login failure, in a root shell on the server, run:
# . /etc/profile.d/kaltura*base.sh
# kaltlog
Then make the request and look at errors being outputted. Once you have these, open the relevant log [most probably /opt/kaltura/log/kaltura_api_v3.log] locate the final error and trace back to where the login flow begun. Paste the stack trace here.
In parallel, also open the dev tools in your browser and look at the errors under the console and network tabs. Once you have the error, we can take from there.