Solution: Unplug the WiFi product, turn it around and plug it back in the other way round. Then the phenomenon is resolved.
Explanation: For wall sockets in Germany, it is not standardized where the phase and where the neutral conductor is wired. The phase via which the current flows can therefore be on the left or right. As the WiFi products from Brennenstuhl have a single-pole switching relay installed, it is possible that a small current is still flowing even though the WiFi product should switch off.