Hmm... they'll most probably be home by now. These alert boxes are really annoying. Is there any other way to temporarily deal with the problem? The ad isn't visible now, anyway. At least not in my browser.
The problem is that their SSL certificate is for www.drivethrurpg.com and the graphics are being delivered from a different subdomain: graphics.drivethrurpg.com. They need to host their graphics on their main domain and not this subdomain to avoid this warning message appearing.