The ending slash does not matter and the URLs are identical, that should not be the problem cause. As you are using a fresh installation of the WordPress and the theme this should be related to the hosting service provider.
I suggest that you install the WP ServerInfo plugin and then activate it. Go to Dashboard > WP-Server Info menu and check the PHP Memory Limit. It should be at least 256, or even better 512. Try to increase it to 512.
The other thing is the PHP Script Execution TIme if it is 30 you need to contact your hosting service provider and ask them to increase it to 300.
The other point is the mod_security rules, the hosting service provider should exclude the builder URLs from that rules. So it should be something like this:
HTTP://www.something.de/x/* this will ensure that all the /x/ folder and subfolders will be excluded from the mod_securoty rules. For more information:
The other point is the version of the PHP, Make sure that it is 5.6+ or 7.1+
Finally, I suggest that you move the same website to your local machine and test the case and see if it works, if yes then you will be sure that the problem is from the settings or a limitation of the hosting service provider.