>I recently found a tricky bug in a c/s application developped with VFP5a.
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>When the user want reset a date cell in a grid to the null value, the program triggers a simple REPLACE vdate WITH .NULL. The vfp environment then blows up entirely on TABLEUPDATE.
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>I found no way to track down what happens. The debugger will not help as VFP really breaks! Curiously this only happens on my NT4 development workstation (with service pack one), not on the series of W95 workstation i already transported the software to.
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>Is it an NT bug that i can patch from MS or a known VFP5 bug... with no patch?

I haven't seen this specific issue (there's far from enough detail to isolate the issues given here) but as a general rule, you need to update your NT environment. At the moment, we're up to SP# (Service Pack 3); we're on the verge of SP4, and there are interrim hot fixes which will be folded into the SP4 release.

There are significant issues addressed by SP2 and SP3, and while i can't be certain that it will deal with your specific problem, I am certain that installing SP# will create a more reliable NT environment.

In addition to the NT Service Pack, you should also make certain that the latest patches have been applied to VFP, bringing VFP 5 to 5.0a build 415 (the patches are available as part of Microsft's Visual Studio Service Pack, which can be downloaded from Microsoft's Web site (it's big, so it may be worthwhile for you to order it from Microsoft on CD.)