I looked into it for hours and finally found out, that the reason is located in my permalinks. As you might see above I have the permalink structure
(so “php” is a subcategory of “it”, the postname ist “fehlerseiten”).
When I change this to e.g. /%year%/%monthnum%/%day%/%postname%/ then everything works fine!
Is this the right place for asking for a bug fix? Or does someone know where I have to fix something for myself? I am new to WordPress.