Thank you for the response!
To your questions:
– Same WP versions (4.9.6)
– BD Version is different because I updated recently on the new site (5.3 on new, 5.2 on old). But with 5.2 it did not work.
– Same theme
– Mostly the same. On new site we have qTranslate installed.
– Same server
– Same PHP Version (5.5)
Best regards