A bit of a wild guess from me: could it be that “Print Message When Reference Is Not Found” is enabled?
https://docs.mendix.com/appstore/modules/excel-importer#references
300 records should be easily doable for the importer, so something must be going on in your case. Enable statistics and see if you can pinpoint a record where it happens. (try 150 records, if that works try 200 etc)
You may want to start eliminating steps in the template: remove all columns but one for example, and see if that works. Then extend to see where it breaks.
I hope this helps.