Hi! I had the same problem with two sites hosted by Strato on the current WordPress version (5.1) – suddenly and without having updated WP or one of the plugins:
Internal server error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator at firstname.lastname@example.org to inform them of the time this error occurred, and the actions you performed just before this error.
More information about this error may be available in the server error log.
The solution is, PHP Boost turn off. This can be found at Strato under Databases and Webspace / Set PHP version. The (then working) PHP version is currently 7.2 for me.