Hi!

From the first look the answer is ready in this lines:

> Out of all the db servers I support, with tens of thousands of GDB files to back up in this
> same manner every day, I've never seen this happen before even once. But on this server,
> it's happening consistently - about 4 nights out of 5.
>
> And it's a different GDB file every night !

Telling true, this leaves almost no chances for something except HW or OS. But provided your problem is well reproduced - please prepare stack backtraces for 2 cases (to be able to compare) and send them to me privately. To get meaningful backtrace use debuginfo for your build.