Unhandled Exception error after completing spreadsheet import
This is usually related to either the formatting in the original spreadsheet or a corrupt map file.
In Excel, copy the sheet you're working with and paste the data into a new sheet using Paste > Values. This will strip the file of all formatting and show any hidden columns. Make any changes that may now be required.
Refresh 4Matrix using File > Open Windows > Close All Windows and start the import again.
If it continues to fail, and you're using a map file, please try again and disregard the map file. As a quick test, try importing just 1 subject column (with your basic pupil details) and check that it works. You could also create a new map file if you have a series with the same subjects using Admin > Edit Subjects > Create Map from within the relevant series.