Is there someone who implemented the DBus Property "Progress" into a Qt-Application? The problem seems the return value isi (Integer, String, Integer).
For all other properties with a single return value like "Operation" or "LastError" the Qt DBus API works fine.
I found an email from 28.Oct 2019 in the mailing list where Bastian Krause had the same issue. The answer was only that he had to cunsult the Qt DBus documentation on how a tuple is mapped to Qt types.

Thats a good hint but after hours of searching the web I didn't find a solution. The return value for a property is QVariant in Qt.

Trial 1 -> Read value by iface.property("Progress"):
This is the error message I got when I try to read the progress property:
Cannot construct placeholder type QDBusRawType

Trial 2 -> Read value by iface.call("Get",...):
Not able to get any information. The arguments of the returned QDBusMessages are always empty!