Dunno, if it is correct: I fell into the same issue. Just deleted the complete folder, then started synaptic again and the folder was new created = issue gone.
Not sure, if this is enough. Maybe the reason is, debian is working on umask settings and (as far as I read), there is normally no "default" setting for umask in debian. It is just led to admininitrators choice. There is also a difference between directly logging in as root and changing to root with su- . This will be fixed in trixie. Hans
