Thank you for your reply.
I changed ‘WP_DEBUG’ to true yesterday because I read this in another thread. As I understand, if I press the generate-button on your tool, the page should be reloaded and the error information should resides on top of the side. My problem is now, that the refreshed side never appears because “The connection was reset”. (The big box with an exclamation mark and a “try again”-button).
Do you know, if I need a unusual server setting, maybe a special PHP or Apache module?
The “themes” directory is writable because the twentyfourteen_child directory was created.
Many thanks in advance.