Either the table has been corrupted, or you're running a 64-bit Texis on a 32-bit database (ie. you recently upgraded from 32 to 64-bit Texis). You can run texis -version and look for -32 or -64 at the end of the second line to see which version you're currently running. 64 and 32 bit databases are not compatible due to the fundamental type change. If so, you'll need to re-create your database and tables from scratch with the 64-bit version, and re-import your data with timport or the like, from a text dump from the old database (using the 32-bit Texis on the old 32-bit database).
If you have not changed from 32 to 64 bit versions (or vice versa), then SYSTABLES has been corrupted. Stop all database activity, and run kdbfchk -o fix.tbl SYSTABLE.tbl, then copy fix.tbl to SYSTABLE.tbl. kdbfchk will attempt to repair the SYSTABLE.tbl file. You may have to use addtable to add some existing tables back into SYSTABLES, if kdbfchk removes some corrupt rows.
