That usually means that you are already logged in to the premium service (mbl services) and you don't need to login again until you clear cookies or your browsing data etc. You need to wait for a little while for the download to begin automatically. It starts within a few seconds most of the time. This is just the lag for the service to bring us the download link directly or sometimes the file is downloaded first to a Mobilism server and then we can download it from there which happens automatically (no additional clicking involved).
I use the premium downloader as I've paid for it and once I'm logged in for a file download, I don't have to login again each & every time I want to download some other file in a post.
Thank you for the answer. However, that doesn't appear to be the case here. Again today, the first download went perfectly after I logged in to Premium. The second attempt on a new book gave me the same grey screen and nothing happened - no download attempt at all. I was eventually forced to go via the dropapk link to get the file. I use a VPN (CyberGhost) so would that affect it?