FreeRDP license is incompatible with gnome-boxes
FreeRDP uses the Apache license, but Boxes is GPLv2+.
Apache and GPLv2 are incompatible licenses, so it's no longer possible to use Boxes under the terms of the GPLv2. An out is provided by the "or later" clause -- Boxes can be used as GPLv3, which is compatible with the Apache license --
Solution: change Boxes's COPYING file, and the license displayed in the About dialog, to GPLv3+
Optional: to avoid confusion, you might want to increase the license version in the source files as well. That's not required.