Looks like Kaspersky just doesn't like the certificate, considering that when this started for you (2 months ago) lines up approximately with when the certificate was issued (Sept 6th, 2017).
Do you log in every time you visit the site? I suspect the error
may have to do with ads, but I don't really know for sure. Some cursory Googling of the issue seems to show that the error is either Kaspersky's fault (people get this error on sites like Google and Facebook) or the fault of external services.
@drp may also have some insight.