This problem does not concern to m2e pro.
The process is rather simple – the image url is sent to eBay in request, the eBay fetches image and store on its server and returns the new url from its server. If it cannot fetch the image for some reasons, it returns the error.
Why this happens? Just a few ideas:
As an other idea, it might be problem on eBay service, for example it is overloaded with the request to its image hosting server, it just starting refuses some requests.