The save fails with this error because the database (index) file for the client is greater than 2 Gbytes. With the Solaris 2.6 release and SBU 5.0.1 this is no longer a problem.
However, with earlier versions of the Solaris software you need to open nwadmin -> indexes -> select appropriate client -> select appropriate fs -> remove oldes cycle -> reclaim space.
You might have to repeat a few times to reclaim enough space. The indexes can be re-created later, if necessary, by using a scanner.