I have two sites (live and staging).
On a live site I use a namecheap certificate.
On a staging site I use a let's encrypt.
I can active Uptimeon staging site, but not on live site.
It is definitely related to your side as I found this in performance cURL error 60: SSL certificate problem: unable to get local issuer certificate.
Also there is a second issue, when you try to acivate Uptime and you get error:
UX does not show any issue. So you're aware of an error but I can't see any error on the page. This is not really user-friendly.