It works. Thank you very much.
I experienced while testing, that when I was too quick with clicking the ‘Set featured image’ button, it brought me to the homepage, when I waited a bit longer, it behaved as expected.
Does this have to do with the order of loading of things?
thank you very much for checking. We usually update our plugins right away when there is an update available, apparently something went wrong with the WordPress notifications. We had 1.0.9 and now 1.1.5, and the error is indeed resolved.
(The error on the registration page is also solved.)