Strange error on Interactive UNIX w/INFORMIX SE

We are getting a error -121 on a sql query to update a table by doing ' begin work; lock table coverage; update table coverage set c_fld1 = "D" where c_fld1 = "E" and c_fld2 = "80" commit work; ' the table lock is 'in exclusive mode' and we clean up the transaction log before we run this query like 'isql db query' and it will lock the table and then say Err -121: Cannot write log file record we have the log permission as '666' and you can see the log file grow from 0 byte to like 2308 bytes (not always this numbers but something around it) and we have the system lock limit set to 300 and the sar report shows only 30 locks was used when this query is run. informix tech supp keep saying this is locking problem, but dont have clues how to correct this. HELP!!! if anybody out there have some experience with INTERACTIVE UNIX that may have any suggestions.