Wireshark 3 extcap_example.py not working on Win8.1: not capturing any "fake" packets

edit

My sytem setup is:

• Win8.1
• Wireshark 3.0.0 (v3.0.0-0-g937e33de) installed
• Python 3.7 globally installed
• example extcap https://github.com/wireshark/wireshark/blob/master/doc/extcap_example.py copied into c:\program files\wireshark\extcap
• extcap-ex.bat created in c:\program files\wireshark\extcap which starts the installed Python 3.7 interpreter with the extcap_example.py, passing all CLI args to it using %*

Wireshark correctly detects the extcap example plugin. The config dialog correctly comes up, but when trying to start a capture, the following error message box pops up: "No packets captured. As no data was captured, closing temporary capture file. [...]".

It makes no difference running Wireshark as admin or not.

What am I doing wrong? Or is the extcap_example broken on Win/Wireshark 3? For reference, the same extcap_example.py works correctly on Ubuntu Bionic with a development 3.1 version of Wireshark.

Any help greatly appreciated!