If I copy a group from a (source) file, I can paste it into another file, but only if the source file remains open. If I close the source file after the copy, but before the paste, Synfig exits, without pasting in the group.
I copy the group from the Layers panel with control-C (Windows). I paste the group into the Layers panel with control-V (of course, after selecting a layer, above which I want the group pasted).
I am running on a Windows 10 virtual machine (version 10.0.19041) with the latest updates, under Parallels 17.0.0 Desktop (51461, the latest version) on an iMac running OSX 11.5.2 with all the latest updates.