now I've cleared up that the problem is not due to absence of ImageMagick, I've taken another look at the error message, and it's not 100% true that it's uninformative, as it refers me to the server error log. Here's the error message:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, email@example.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
As for the error log, it's basically two items repeated: the error
and the missing 500 page:
I didn't install the Gallery debugging module as I doubted my ability to make use of it!