I like to do this job.
500 internal error cause by couple of issues, first we have to look into your server specification, even though you mentioned it's working fine,
recently my client also faced same issue and their hosting provider said nothing wrong with server resources, but i request him to increase php memory and boom error gone,
more over if plugin is not compatible with theme, and wp it'll not run, until you fix the issue whether in plugin or theme.
another reason is the misconfiguration in internal settings.
so if someone telling you the solution without looking into system, he probably making you fool, so stay out of scams and choose the freelancer wisely,
in my part i can assure you if work will not done i'll not charge any money, thanks