Check to see who is using the rbs whilst performing the export. I would expect that this will be the user doing the export.
The next thing to do would be to pass a table list (via the export parfile) and therefore only export the tables not the snapshots, see if this stops the error.
I am not sure if this is a valid workaround or not, but the snapshots are just copies of data elsewhere so why should they be backed up???

Have Fun