We are having this same issue but our columns appear to be formatted correctly. The behavior is strange. We can sometimes import in a small quantity of 1-10. And sometimes we cannot. The file is checked and there is no data but I cannot figure out why the issues persist on some products and not others.
We are migrating off 1.9 to 2.4 but until the new site is done, we need to have this completed for the migration.
If anyone is able to provide some insight I would appreciate it and I will send you a $50 Starbucks Card if you can figure this out so we can import these. Thank you in advance for the help!