I may have just solved that issue:
Memnon wont finish your mission Skits, because the HE-Memnon (on Primus Planet, with beautiful speech files) wasnt even the mission giver.
Mission came from the SHE-Memnon on Arx Tiberius, Tarsis (also a Gatekeeper by profession she is...).
And SHE will also let you turn in the stuff again.
It somehow feels like even the contentDEVs in Progen space lost a bit track of what is going on there?
One more thing though:
I am not sure if that NPC is there in general or if I just accidently summoned her to Arx Tib by misusing one of those strange tokens...