There’s two separate issues that we’re aware of, causing the same error with Dalaran Heist. One of the issues should be fix today. Our team is still looking into the other cause of the Dalaran Heist issue, which looks to be affecting everyone still running into the Dalaran Heist error.
We’re looking to get this fix pushed to everyone still affected at a later time. We’ll continue to update and monitor the issue for now. Sorry for any confusion and hopefully we’ll have more to share soon!