Unfortunately Mixpanel is deprecating their JQL API and removing it completely in a couple months. I'm guessing they may have started turning it off for some accounts already based on the error you're seeing. As an alternative, you would have to sync Mixpanel data to a warehouse like Snowflake or BigQuery to continue using it for analysis within GrowthBook. We're working on some scripts to make this migration easier, but they aren't quite ready yet as we assumed we still had a couple months left before needing it.