(Some time passes...)

Sorry! I did warn you it slowed things down:)

On the basis of what you have posted, I'd have to concur. It may well be that the 'fix' for 5.6.0 would be to downgrade your copy of DB_File rather than unpgrade, assuming the earlier version is still available, but if upgrading your build to 5.6.1 works, that is probably the easiest and safest route. There were lots of things fixed between 5.6.0, and 5.6.1, and your bug could well be another manifestation of one of those fixed bugs.

You might try searching the changes file and bug lists for 5.6.0/5.6.1 and see if anything there looks like it might be your problem, but even if you found it, all you would know is that you had to upgrade to get the fix (or possibly where to obtain a patch so that you could re-build 5.6.0 to correct that one bug), but in the end, if the upgrade does the trick, knowing why it does is just icing.

Good luck.