In version 6, we've moved to a newer .NET version 8 (used to be v4.8). It's a substantial change, so it may take some time to iron out issues like that. We will also add a test for 600 nested if functions to ensure this problem is captured before releasing the software.
Going forward, I'd suggest using lookup tables instead of doing lookups with IFs. Keeping lookup tables in .dset files also makes them re-usable. Besides that, .dset files can be edited in EasyMorph - we've added Dataset Editor: