The Site24x7 Windows server monitoring agent has been upgraded to version 19.0.5 & above.
Atleast one TLS version, either TLS 1.0 or TLS 1.1 is present in that machine and https://plus.site24x7.com/ is accessible in IE from that machine.
You can also try a work-around for enabling client-server data communication, if TLS 1.2 version is not available in your 2003 and 2008 Windows servers. Learn more.