Magento – Exporting Products with Category Names Instead of IDs

The file you generated through Magento Export (whether this is default import/export module or default dataflow profile) is NOT going to work if you import it back, because it doesn't preserve the format.

If you want to update weights, I would recommend this:
1) filter your CSV in Excel, filter only those which have "sku" field assigned
2) copy-paste the filtered rows into new spreadsheet
3) leave only 2 columns - "sku", "weight"
4) import it back.

I'm pretty sure it's the easiest flow to solve your concrete task.

For those who are looking for tool for Magento that allows both export and import while having similar format for both routines, check out this one.

I have seen similar behavior coming from M2EPro imported listings. What it seems to do it drop the shipping field data. So I just sorted the grid by weight, found any empties and corrected the shipping weight. The error I got was:

Notice: Undefined offset: 3347 in /home/builders/public_html/app/code/core/Mage/ImportExport/Model/Export/Entity/Product.php on line 875

Note the offset is actually the products internal magento ID, so your offset of 103041 is the products ID, edit and resave that item in the backend and try to export again... The export should continue to the next record. Worked for me.