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Excellent job :)

You mis-understand.
That it "works" on a single machine is basically down to luck as much as judgement.

When you add new code, you have a chance of exposing previously hidden bugs.
When you...

Are you sure nnodes is initialised?

If you do this (and only this) in a new program, does it work?

The problem is almost certainly down to some mis-use of some memory you've already allocated....