The TSO Mission system is, how shall we say, VERY buggy at the moment. Especially if someone shares a mission with you. Some people are getting totally locked out of missions if someone shares the Double Void Shard mission from Moors of Ykesha and they complete it. Quest Sharing is often not giving people the items necessary to do the missions (Holy Water, etc.).
All of this is being investigated by the Developers according to Kiara:
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