Hi,
I am assuming we're talking about a CE installation, correct? while making the request, run:
# kaltlog
from the server's shell and look at the errors. A blank page means the request is not coming back correctly, probably resulting in HTTP 500. In parallel to running kaltlog, open your browser's dev tools and look for requests that ended with 40.n or 50.n.
Then, go to the relevant log [usually such errors will be logged to either /opt/kaltura/log/kaltura_api_v3.log or /opt/kaltura/log/kaltura_apache_*errors.log] and look at the lines leading to the final error.
Feel free to post them here [masking any sensitive data, of course] if you need more help.