I have the same issue, although it has been working fine until now. Luckily there are not many changes I need to update the site with!
I am importing & merging products, with an image column pointing to an external URL, although the URL is not really external at all, as it is in wp-content/uploads/product-images – I just couldn’t get it to work properly just specifying the image file name & placing in the year/month folder etc.
Will restarting the server help? At least for now, until the issue is prolerly resolved?