Click the Cog on the Battle.net Launcher
“Scan and Repair.”
GG. Worked for me, haven’t had issues since.
Let me know if it works for you.
EDIT:
As of now, it appears this no longer works…
Instances are bugged still.
Was working for a bit, but then bugged out.
So no idea.
Seems to be working better for me!
Yeah fixed all the timewalking problems for me as well. 
worked as expected.
Glad I was able to help.
Tried and no change. Unable to purchase anything from any vendor or enter instances.
Did not work; still “D/C’d” after looting last boss in TW Legion dungeon.
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Nope did not work. My game broke after doing a WQ in Ahj Kahet. Can’t log in again 