We have identified the cause of this issue and have deployed a fix. We are monitoring the service.
Posted 17 days ago. Jul 31, 2017 - 16:10 UTC
We are investigating an issue with Calendar Service group meetings scheduled by using @Spark. If the user clicks the link "Join Cisco Spark Meeting" in the calendar item, an error is returned. This issue does not impact 1:1 meetings scheduled with @Spark.
As a workaround, join the meeting by clicking the Join button in Spark instead.