ACE report - Help??

I have a simple ACE report program which runs twice each day, at 07:03 and 13:03... It selects from two tables and formats the info - nothing clever or complicated. The early run invariably (well, for three days running) fails with the following message. Cannot read system catalog (systables). See error number -211. The lunchtime one runs fine. From my reading of the error manual, this would seem to be generated from the DATABASE section of the ACE program. The manual tells me that this can occur .. but not why. The database is updated around the clock by processes feeding it data from ascii files arriving from remote sites via x25. All user processes only inquire. We run a backup (tbtape) at around 04:30. I can find nothing which happens to the database at around 07:00. Versions? Online - 4.00.UH1 SQL - 4.00.UE2 The database is less than 800Mb in size. Heeellllp! Can anyone point me at possible reasons for this behaviour? ---------------------------------------------------------------------