thanks for your reply. That’s actually how I figured out it is Yoast.
I can reproduce it, if I turn off Yoast it’s working, if I turn it on it’s not. (if German is active)
I just checked with the firefox console and got the following error message:
The character encoding of the HTML document has not been declared. The document will be presented in some browser configurations with garbled text if the document contains characters outside the US-ASCII range. The character encoding of the page must be declared in the document or transfer protocol.
I guess this is why the problem occures. When Yoast is activated there is an additional menu point on “appearance -> customize”. As this is in the backend question is is this a problem of your theme or of Yoast.