The problem seems to be caused by having two network connections and the corresponding IP routes in parallel:
- WiFi connection to the THETA
- Mobile connection to the Internet
The THETA app seems to take the route via mobile connection to the THETA which of course fails. The workaround is to disable the mobile connection.
In earlier versions of the THETA app & firmware and Android, this setup worked without problems. This allowed to get an image from the THETA and to publish it on an Internet site (theta360.com or whatever) in a single step. Unfortunately, this is no longer possible.