I use Manage WP to monitor my site, which uses Site24x7 to monitor uptime. Manage WP told me I should reach out to Let's Encrypt to help with this error: "Connection was reset remotely during SSL handshake" which is causing the uptime monitor to disconnect.
Although you can see errors reported, the errors are relatively minor and I don't believe that any of them are relevant to what you're seeing with the Site24x7 monitor.
Is it possible to get some more information from Site24x7 to help investigate?
In particular, does this error occur every time or just occasionally? Can they provide any further technical details about the connection error?
It seems like this particular error would typically be caused by a firewall or other anti-abuse system run by your hosting provider (which somehow decided that some of the connections from Site24x7 were abusive, excessive, or forbidden). Can you ask the hosting provider whether this could be the case for some reason?
I don't believe that there's anything wrong with your certificate or your HTTPS setup itself—the tests that I linked above (and my own browser-based testing) indicate to me that these configurations are actually OK and not particularly problematic. (The error messages that appear in these tests could point to potential configuration improvements, but again, I don't believe they would be directly capable of causing the specific error that you reported from Site24x7.)