We will try to make you a testing environment in a few days.
Right now I experience other bugs outside burn engine. So there is actually no reason for me to use other PHP versions other than the default one in vestacp.
So my tracker for this bug is now closed and no reason to get fixed since other features are affected as well.
Internal Server Error
The server encountered an internal error or
misconfiguration and was unable to complete
Please contact the server administrator at firstname.lastname@example.org to inform them of the time this error occurred,
and the actions you performed just before this error.
More information about this error may be available
in the server error log.