Import of contacts failed after deleting all

Import of contacts failed after deleting all

I succesfully imported 4 batches of 1000 records xls files.
The fifth batch refused, indicating it shouldn't have more than 301 records.
I delete all contacts to start again, with that 5th f batch but alas,
now all files, including those that imported previously fail with the same cryptic error.

How can I get all of my records imported?
BTW, the mass delete of contacts is really combersome.

Please help.