Please help with the export/import tool!

Hi all! i"m triying to build a small store with something like 2000 products and i'm using the export, import tool for the job. But on the last import, i got an error...

Uploaded file is not a valid spreadsheet file or its values are not in the expected formats!

And if i take a look at the error log, i get this:

Export/Import: Invalid header in the Products worksheet

Sometime ago, out of the blue i started getting that pesky output error:

Fatal error: Out of memory (allocated 321912832) (tried to allocate 95 bytes) in /home/socialb/public_html/system/PHPExcel/Classes/PHPExcel/Worksheet.php on line 1176

And in the error log

PHP Fatal Error: Out of memory (allocated 321912832) (tried to allocate 95 bytes) in /home/socialb/public_html/system/PHPExcel/Classes/PHPExcel/Worksheet.php on line 1176
PHP Warning: Cannot modify header information - headers already sent by (output started at /home/socialb/public_html/system/PHPExcel/Classes/PHPExcel/Worksheet.php:1176) in /home/socialb/public_html/admin/model/tool/export.php on line 49
PHP Warning: Cannot modify header information - headers already sent by (output started at /home/socialb/public_html/system/PHPExcel/Classes/PHPExcel/Worksheet.php:1176) in /home/socialb/public_html/admin/model/tool/export.php on line 50


But since i never really used the export part of the extension it really didn't bother me that much. But today after i tried to upload new products, i get the above error....

This one to be exact:

Uploaded file is not a valid spreadsheet file or its values are not in the expected formats!

And if i take a look at the error log, i get this:

Export/Import: Invalid header in the Products worksheet

Can anyone take a look at my xlsx and maybe give me a hand? Please? I'ts my first Store, and i really don't know anything about php and stuff like that...

Thank you in advance!