Thank you, Jane !!! I think your signtool.exe versions 6.3.9600.16384 is
what makes the difference here.

The older signtool.exe version 6.2.9200.16384 works fine on Windows 8.0,
Windows 8.1 and Windows 10 (timestamp with a 3161 server, SHA-2 and dual
SHA-1/SHA-2 signing). Perhaps Microsoft fixed it with 6.3.9600.16384.
We'll have to check this.

Friedrich