If I grant the WSUSER account site admin priveleges this starts working, take it away and it shows the above. Not sure what permission I'm missing because the user has all the perms required to run the web service (and there are no permission errors during run).
Having the exact same issue with the same workaround.
This is caused by a permissions check within the Web Service call that never appears to return true for an external web service user.
Have logged as MDL-47783
Yours
James