Good news.
The issue discussed in this thread has now been diagnosed and solved by David
Hess.

The problem turned out to be this:
when a persistent instance becomes temporarily unreachable during the period
that an incremental pack is in progress, the object's record might be left out
of the
newly packed file. All subsequent attempts to access that object, including
subsequent
packs, would get an exception.

This rare trap has existed for years: for all versions that supported
incremental packing.

The patch will be included in the next release (expected soon).

Thanks, David, for your good work on this.