The connection to the server was reset while the page was lo

[MC9000]

I'm getting the following message on all browsers (or variation of) when trying to connect to my domain.
"The connection to the server was reset while the page was loading."
For a while now, I've been connecting via a VPN (via an internal IP address) and didn't notice that the "https://mail.mydomain.com:8100" and "https://mail.mydomain.com:9100" haven't been working (not sure when this started) until a client mentioned it. Then I checked, and sure enough, it's no longer working.
I verified that the SSL certificate has not expired for mail.mydomain.com and can connect via SMTP/POP3 fine, with no errors, it's just that the web interface is giving me this error.
It is an older version of AMS (2.72), so I figured something changed in all the browsers (maybe an Internet-wide change) to prevent viewing (weirder, though, I can connect via an internal IP address even w/a cert mismatch and ignore the SSL warning, though I can't do that with the legitimate domain name).
I'd like to convince my client(s) to upgrade to the latest and greatest, but kind of hard to convince them when things suddenly stopped working (while they were working fine before).

Any ideas?

[Code Crafters]

It might be that you're using SSL version 2 or 3 which isn't really supported by any modern browsers now.

We recommend upgrading to Ability Mail Server 4 via https://www.codecrafters.com/AbilityMailServer/BuyNow which uses TLS v1.0 as a default but also supports up to TLS v1.2 and in a future update will also support TLS v1.3. Even TLS v1.0 isn't really recommended for security but is the default for maximum compatibility. The higher up the version, the more secure but less compatibile with older browsers and email clients.

The last update for Ability Mail Server 2.72 was in March 2011 so is very out of date in terms of SSL and not recommended.

Ability Mail Server 4 has many new features including a totally rewritten modern and responsive WebMail that looks way better than the old one and resizes to mobile / tablet screens as well as desktop. Please see https://www.codecrafters.com/AbilityMai ... ateHistory for all updates.