Hey Fred,
This issue is highly likely not caused by the theme. I’d like to recommend first that you try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If that does not help, switch to the default WordPress theme and if the issue persists, there could be some setting in your server that is causing the issue. In this case, you will need to direct this to your web host more over that 500 Internal Server Error is server related. At this stage also, we now know that it’s really not an issue with the theme.
If the issue is fixed by switching to the default theme, please switch to the parent theme (if you’re using a child theme) then test again. If the issue persists using the parent theme, please give us WordPress Admin and FTP access in a Secure Note.
Thanks.