There was a change last year in the nwb and as far as I know now they use version control for reading/writing nwb files. Meaning that if a file was created with a specific version, you have to use "generateCore" from that version as well.
The only solution that I can think of, is have a cache of all versions and call the appropriate one depending on the file that needs to be loaded.
So just supporting a single version would unfortunately not be a solution.
Here are the versions:
https://github.com/NeurodataWithoutBorders/nwb-schema/releases
I can take a look at it if you still have trouble the week after the next one.
Konstantinos