Problems with system catalogue

Frequently, some of my apps issued the next message error: Cannot read system catalogue (systables) File is not open. Does anybody know the cause of this error and the solution. The apps are written in 4gl. Most of the time I re-execute the application and the message is gone, but not always. Thanks for your help. Alfonso Trevino alfonso@mtecv2.mty.itesm.mx P.S.: This error message appears no matter if we use the standard informix engine or the On-Line engine.