Hi there,
It seems that the problem is not related to our plugin if you still have it after deleting the plugin folder.
The plugin doesn’t store any options or make any changes to the database tables. If activated it shows the information. To uninstall you can simply delete the folder as there is nothing stored on the WordPress installation, neither files or data in the database.
When deleting the plugin folder /server-ip-memory-usage/ everything related to the plugin is gone.
Sorry that we can’t help you to solve the 500 error. Did you install anything else or make any other change?
Best regards from Spain.