Error 90012 Missing SKU on uploading a Listing Loader template

For last few days I have been getting this error whenever I attempt to upload product.
90012 Fatal The required sku field is missing from the file. To correct this error, download the template again and use the new copy, or insert the required field into your existing file.

Now the SKU field is definately there and so is my SKU value

The product already exists in both the Amazon catalogue and in my inventory and I am just attempting to change the price. I am using correct SKU and ASIN (checked and re-checked multiple times) and I have downloaded latest Listing Loader template.

The error is usually also accompanied by a warning that another field is not valid, and again tells me to download latest version.

It’s driving me potty - I have tried uploading a Tab delimited text file and also an Excel file and get same error.

The error is only affecting my FBA products. I can change details of my FBM products via an upload using same template but not any of my FBA items.