Same thing happened to me, first i picked up the quest from xur on my hunter, but I wanted to play on my warlock, so I took the quest from the quest kiosk, did all the steps, to find out banshee only has his vendor screen… Then I went back on my hunter and it said to go get the key from Xur. Went to xur, and found out I couldn’t get the strange key again because I already used it on my warlock. So now it is impossible to complete my quest and get the new exotic sidearm.
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I think something like this happened to me too… Started the quest on warlock. Switched to titan for the starhorse bounty step. Went and scanned the thingy with the key. Would let me get the weapon from the box after jumping puzzle.