Ultrasound has detected that the volume where the FRS database is stored has less than 256 MB free. If the volume fills up, replication will stop and FRS event ID 13511 will appear in the File Replication Service event log.
Free up some disk space on the volume containing the FRS database. If this is not possible, move the database to a larger volume with more free space. For more information about how to move the database to a larger volume, see article 221093, "How to Relocate the NTFRS Jet Database and Log Files" at http://support.microsoft.com/?id=221093.