I am unable to replicate this issue @ your site, I was able to upload a file just fine. Can you please provide me with step-by-step instructions for how to replicate this issue @ your site?
the button "Add Files / Datein hinzufügen" only opens the dialog to locally select a file.
As you can see on the screenshot, the button is not displayed as usual.
Are you using an ad blocker or a NoScript addon in your browser? Can you please try this in another browser that does not have any extensions installed?
I am not seeing that view @ your site so it sounds like it might be a browser issue at this point.