This one was very hard to find. Thanks to a tester who found the reason in the end.
- Use Windows 10 in a virtual machine and give it only one CPU thread.
- Create a local folder e.g c:\temp and put two files into this folder.
- Enable Network sharing for this folder
- Mount the shared folder as a network drive
- Encrypt the two files in the folder and keep Archiving selected.
This is totally weird because if you encrypt the files using local paths it works. Or if you give two threads to the machine.
It also works if you just use gpgtar on the command line. Even if you call it like Kleopatra with --skip-crypto using file names and pipe that to gpg.