After correcting the problems with your import file layout I was able to repeatedly import it without any problems. Specific problems that must be corrected:

Error at record 45. Error did not always occur at the same place. Error not specified:

Correcting the Terms Code from " C.O.D." to "C.O.D." corrected this problem.

Error at record 364 (row 727) invalid customer number. Add missing customer number.
Error at record 452 invalid customer number. Add missing customer number.
After this I correct all missing customer numbers without noting the specific record number.

You must correct your process so that the issues noted above do not reoccur. If you send me a fresh output I can confirm that it is correct (or not as the case may be).

Please note that the Customer number and Terms code are validated fields and must be absolutely correct for all records. For the terms code issue I must assume that the extra space causes confusion rather than an out right rejection of any particular record.

The NULL Due date and Discount Date in the template do not appear to cause any problems but I would not want to guarantee this. It may be safer in the long run to include these fields in the import file (by populating them with the invoice date) rather than relying on including them in the templates.