Seamonkey 2.53.6 doesn't open swissdelphicenter.com

[Geronimo's Cadillac]

Seamonkey 2.53.6 doesn't open swissdelphicenter.com.
"Secure connection failed. The document doesn't contain data."
Why? I have no idea.
All other browsers do open this page.
OS = Win 7
Test on another system with Win 10: the same result. All browsers open the page except for Seamonkey.
The issue is not related to any antivirus software.

Please help.

[-Px-]

Confirmed, seems something with certificates handling, but no messages in network/error/browser console, so it is not clear what exactly is wrong

[TPR75]

Seamonkey 2.53.6 doesn't open swissdelphicenter.com.
"Secure connection failed. The document doesn't contain data."

I've enabled logging in "about:networking". There was need to make some workaround to make it log:
https://developer.mozilla.org/en-US/doc ... TP_logging

In log file are such errors:
D/nsSocketTransport ErrorAccordingToNSPR
D/nsSocketTransport nsSocketTransport::RecoverFromError

Could be a bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1563394
https://bugzilla.mozilla.org/show_bug.cgi?id=1391543

[TPR75]

It could be this bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1434137

If yes then it requires more work because it looks like it wasn't implemented yet - after file "1656697-PARTIAL-v2-78.patch":

Only partially applied because of missing Bug 1434137 "Implement websockets over http/2"

Well, maybe in some next version of SeaMonkey it will be fixed.