ISAM error: illegal key descriptor (too many parts

IDS 11.50 FC2 HP UX 11.23 Noticed below errors while running onchecks on a couple of databases. Dropped and re-created the indexe. Would like to know the cause for these bad index pages. Data in these tables are 10 years old. HP confirmed no hardware issue. Everything seems to be working fine for few years and then we come up with these bad pages. Had a crash with one of the primary chunks going offline couple of weeks back but again could not identify the issue based on the stack files. Validating database dbew48 Validating systables for database dbew48 ISAM error: illegal key descriptor (too many parts or too long). TBLspace name wq_hist_t Owner informix TBLspace number 40065a Tabid 185 Row size 40 Number of columns 8 Number of indexes 4 Number of rows 1098290215 Date created 09/09/2012 TBLspace major version number 189 TBLspace minor version number 1514 TBLspace type T Locking level R Number of pages used 1092619864 First extent size 4386448 Next extent size 731074 Flags 0 Site Database name Validating syscolumns for database dbew48 Validating sysindexes for database dbew48 ISAM error: illegal key descriptor (too many parts or too long). Validating indexes for dbew48:informix.wq_hist_t... Index wq_hist_x01n ERROR: Bad key information in TBLspace description. ERROR: Index wq_hist_x01n for dbew48:informix.wq_hist_t is bad.