We were looking into a similar issue and it’s actually down to a few things but mainly the fact you like us are trying to use PHP7 and that mysql-connect (https://php.net/manual/en/function.mysql-connect.php) is no longer part of it.
HyperDb as it stands is written to use mysql-connect not mysqli-connect so you will get the 500 error and due to a bunch of error suppression no actual error.
We are currently evaluating this fork with mysqli support to see if it suits our purposes.
https://github.com/soulseekah/hyperdb-mysqli
