There is a bug with Informix 64-bit on Solaris 8 that causes Informix problems with failed backups. Since this is an Informix issue, Legato's compatibility guide/manuals+supplements do not list 64-bit Informix on Solaris 8 as compatible; 32-bit Informix on Solaris 8 is supported.
This was true as of April 2001 (Even after Informix introduces a patch, Legato will need to retest, of course).