UPTime is not being enabled

Uptime and wp-checkup are not working for a member, I am getting:
> Your domain does not appear to be publicly accessible on the Internet (SSL certificate problem: unable to get local issuer certificate - 60). We can only scan publicly accessible websites.

But the site is up and I have also checked with https://geopeeker.com/ and https://www.uptrends.com/tools/uptime, the site is indicated up there too.

The error log is clean, just HB debug information: https://pastebin.com/ya9A0LKT

Any idea, what could be the reason?