I dug a little deeper. I am reasonably certain that my ensight geo file is fine according to the documentation.
I wrote a small routine that reads it, and does some plausibility checks.
However, running ens_checker tool on that file, it claims that some of the node indices are negative. Which is not the case according to my check routine. Weirdly enough, ens_checker reports the first error as:
For hexa8 element number 67108864:
connectivity element 6, is given as -144802628
This is beyond the number of nodes (373714618) in this part
67108864 elements x (8 nodes per element) x (4 byte per node) = 2147483648 byte = 2^31 byte.
In the documentation for ensight gold file format, I found no mention of such a limit. And I would expect that ens_checker would warn about the amount of elements being too high, if elements x nodes exceeds such a limit.
Any ideas would be welcome.