I need help with php script install (http 500 error)

I purchased a themeforest php script. Appears to work but once it reaches the activation part where I am required to enter my purchase code I get a blank 500 http error.

I need this investigating as the owner of script is unwilling to help, shouldn't take long for PHP expert.

It imports all tables to my database as normal but then after entering purchase code it fails, see attached.

will provide all details.