A 500 error is a server error. Either PHP or SQL is crashing. Enable debug mode and maximum error reporting in Joomla global configuration then retry to see if error is exposed. If not check server error log. If issue persists begin unpublishing non-core CB plugins 1 by 1 within CB > Plugin Management to see if it's something installed. Beyond that there's nothing more I can suggest as you'll need to debug your server error.
CB version is 4.0.0 rc2
There is no CB 4.0.0 RC2. Latest release is 2.1.3+build.2017.09.13.16.14.19.ba9d1f686.