The Xenology quest hasn't been appearing for weeks now.
For a while it worked, and at some point I accidentally abandoned the quest (the quest was complete, and just needed turning in).
Since then, for about 4-5 weeks in a row now, I haven't been able to pick up a new Xenology quest from Xur; the quest simply doesn't appear.
I would appreciate any help with this.
this is a known bug. you should be able to pick it up on another character, complete it turn it in, and this should grant you access again on the original character.