Webview in BasicUI without content

I have some embedded websites:
Webview url="" height=17 icon="none"

These are shown perfectly in the Android App and also in the iOS app. But with the BasicUI (in Firefox) the box is empty.
The only log lines I found are:

17-Dez-2019 20:20:31.593 [DEBUG] [basic.internal.servlet.WebAppServlet] - Servlet request received!
17-Dez-2019 20:20:31.683 [DEBUG] [basic.internal.servlet.WebAppServlet] - reading sitemap default

First time I observed that behavior with 2.5.0 M5 (now I’m at 2.5.0). In former versions I had no problems, but I don’t know when that has changed as I’m using theses embedded sites only randomly.

Any idea how to solve it?

Have you tried clearing browser cache?

Yes, I did!

The behavior occurs also in Firefox on Android.