Hey Anil, welcome to the Community!
At first glance, this sounds like an issue with what protocols and ciphers that .Net supports. This is typically the problem when something like this happens - maybe it used to work, but with no apparent changes on your end it stops working. As more new ways to break encryption are found, older protocols are switched off across the web in favor of newer standards.
I believe that's what is going on here. A Microsoft article describing TLS practices notes that the most recent versions of .Net will use the TLS provided by your operating system. The article also notes that .Net 3.5 isn't great for TLS support:
For .NET Framework 3.5 - 4.5.2 and not WCF
We recommend you upgrade your app to .NET Framework 4.7 or later versions.
The article contains information for potential hot patching - some but not all distributions of .Net 3.5 can be patched to allow TLS 1.2 connections.
If there's a question over what ciphers and versions of TLS are enabled, the best way to find out is by checking! Any static answer about what versions are currently supported will get outdated quickly. I like to use SSL Labs to see what a site does or doesn't support currently. The results of a test there show you what versions of TLS are enabled and try to show you what particular Java versions (no .Net unfortunately) will have issues connecting.
Cheers,
Daniel
P.S. - although I just told you to test, at the time of this answer, only TLS 1.2 is enabled on my Jira Cloud instance. This means that lack of TLS 1.2 support in .Net 3.5 is most certainly the problem you've encountered.
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