This is what Firefox tells me when I click on a Bookapy-Ad underneath a story, and it tries to send me to BookapyX:
Websites prove their identity via certificates. Firefox does not trust this site because it uses a certificate that is not valid for bookapyx.com. The certificate is only valid for zbookstore.com.
I can then click on "Advanced", "Accept the Risk and Continue", and confirm that twice more to still visit the site. But I kinda feel like that's a massive hassle, and many people will be put off by the warning.
EDIT:
Could it be that ZBookstore is going to replace BookapyX, and the ads on SOL are simply not updated yet?