The easiest diagnosis in this case is to enter a couple of records into the AppxIO data for the file with the problem, then export it and run it through 'appx -e={filename}' to get it into an ascii readable format. Then, run the problematic Portdata/{filename}.dat through 'appx -e={filename}' (renaming as needed). Then, compare them against each other, comparing the ascii data format Appx was expecting to receive (the data just entered) against the ascii data you were attempting to import.
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How do I upgrade APPX 3.5 or 4.0 to version 4.1?

To fix this problem, scratch all the Design file "./data/*/*/Struct/" directories on the machine you're trying to import from *EXCEPT* the 0-app /Struct directories. Then run 'File Create' on all those Application Designs, to recreate those Struct libraries. Then re-export your "import from" machine's Design files, transport them to the "import to" machine's Portdata directory, and try the import again.