Hello, I am new to the platform too and when I first installed a few days ago, I also received a 500 error.  It had to do with some php settings, which are noted, along with some other requirements on Github.  I would link directly to the page, but my permissions on this site are not allowing me to share links.  So, go to Github and search una, click the first result, then click wiki in the top menu, scroll down and click requirements (under the getting started section).

I had to dig around for a while before I figured out what I had to do in Cpanel so if you need further detail, please let me know.

I'd start with your htaccess file, what version of PHP are you on? In your cPanel, you should be able to see some error logs. That would help us.
Permissions to change should only be the ffmpeg.exe for processing video.