Was about to finish the Magnum Opus quest and get the forerunner from Banshee, was on the screen where you pick it up but found out I needed more space for the weapon. Left the screen, made more room in my primary slot, and then could not get the screen to come back up. It says to talk to Banshee but it just takes me to his vendor screen. This might be because my other characters have the quest and for them it’s still staying get the the strange key from xur. Either way I’ve tried just about everything and even abandoned the quest line but that made it worse trapping me on the 3rd step with the Stange key.
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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.