Mike,
It looks like server doesn't have access to some images. The following steps may help you to troubleshoot:
-- Check the user that runs ArcGIS Server (you can either check windows services, or view the user that runs arcsoc.exe in taskmanager)
-- Check source raster paths (if you're not sure about the path, you can attempt to repair mosaic on mosaic dataset's context menu, and see the paths -- don't do reapir unless you need to)
-- Check permission on source raster folder, make sure the user that runs ArcGIS Server has access, grant that if not already.
