Most commonly this error can display in the event that a user has attempted to sync with an individual Merus Case case, but the status for that Case is currently set to something other than "Open" or "Stipulation".
How to Resolve
To resolve this, in Merus Case, set the case to one of the status's listed above and attempt the sync again.
Once the case is created in Case Status, adjustments to the status listed in Merus Case can be made without any issue.
Cases closed in Merus Case will also close in Case Status.