Experiencing this issue as well, though SKU's don't seem to be the cause (our SKUS are in the format of 'FOO-####'), so none of them are anywhere near 64 characters.
When I select the csv file and select check data, it passes the check fine:
Checked rows: 1718, checked entities: 47, invalid rows: 0, total errors: 0
File is valid! To start import process press "Import" button
After selecting import, however, I receive this error:
SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`magentoss`.`catalog_product_option_type_value`, CONSTRAINT `FK_CAT_PRD_OPT_TYPE_VAL_OPT_ID_CAT_PRD_OPT_OPT_ID` FOREIGN KEY (`option_id`) REFERENCES `catalog_product_option` (`option_id`) ON D), query was: INSERT INTO `catalog_product_option_type_value` (`option_type_id`,`sort_order`,`sku`,`option_id`) VALUES (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?), (?, ?, ?, ?)
After checking the product listings, it does seem as though the file was uploaded, as all my data was updated, so not entirely sure what's going on.