So, i joined a friend that was doing the vexcalibur mission " //node.ovrd.AVALON// " at this time i didn't know what it was.
When i joined them i instantly got the quest added to my quests. i indeed got vexcalibur at the end and everything looked normal even after we bringed our weapons to the war table like the quest said.
The quest continued and asked to kill some vex on neptune to get a mod, i added the mod to vexcalibur that allows you to pass vex portal to get a chest at the beginning of the exotic mission but after openning chest, the quest totally glitched.
My friend told me that we needed to go to the relic on mars to continue the quest but i was unable to see the quest on my side. we then headed to mars and i saw nothing and even worse, the relic is all glitched asking me to get extensions i already have, and for weapons i own.
[u]to avoid making your game glitched, avoid doing the vexcalibur quest for the moment. at least do not join a friend in the middle of it like i did or without having finished savathun campain[/u]
Note that i bought the two last extensions a week ago and am at the very last campain mission of lightfall but didn't start savathun campain yet.
hope it'll help you, hope i can maybe be helped too