This is the exact same problem I'm having right now. My code used to work and after upgrading to version 14, GetNextSerialValue is returning NULL. I've not forgotten the quotes on the field and like the previous posted above, I created a test database and the exact same code works there but not in the solution I'm working on. Please help.
Thank you for your post.
If this is working in a new file, then delete the script step with the GetNextSerialValue calculation and recreate it. Make sure to include the file name and the full fieldName that includes the table name.
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