wordpress err_connection_reset in Chrome from specific country

I volunteer supporting a news website in Russia, which was hand-crafted in PHP back in 2002-2004. Needless to say, I was super excited when editors hired some folks to build a new version, based on WordPress. The old site is running on mydomain.press. I put the new WordPress version, which is meant to replace the old one, on subdomain.mydomain.press.

And there's a mysterious problem with it.

When an editor is trying to access the site at subdomain.mydomain.press, her browser (Chrome in Russia) instantly reports err_connection_reset, in 9 cases out of 10. Not spinning trying to load the site - an instant error is reported. On my machine (Canada) the same website opens no problem. Well, a little slow (hence I mentioned she's not even seeing the delay - the error is instant), but it opens in 10 out of 10 trials. When her Chrome gets the content (in that 1/10 case), it also shows a slight delay. Only the error case is instantaneous. The old site at mydomain.press is opening 100% of the time.

Connecting remotely to her Windows machine (I'm using Mac OS X) via TeamViewer, I did observe the behaviour described when using Chrome. Interestingly enough, IE didn't show this problem - it loads consistently, except that once in ~10 reloads the page loads with a garbled styling. As if some css isn't loaded properly (but not in a way that would make it an invalid document, obviously).

I'm completely out of my depth. I tried disabling her Windows Defender to see if it's the culprit - nope. I've tried to reset her IP address (as suggested by the same page which offered the earlier way to try and fix the err_connection_reset) - no dice.

I'm not seeing either error from my own Chrome, nor the garbled css (though I didn't try with IExplorer from Canada).

I know they had some ISPs in Russia block them (silly political reason, AFAIK) in the past - but this doesn't look like blocking; she'd be 100% unable to view it otherwise. She's not under any firewall (nor is the website).

what else... nginx is the server used, the setup is "basic", I suppose (I'm not that proficient in configuring it to try anything fancy).

And to make things even more mysterious - the website at mydomain.press (the old version, php-hand-crafted) is opening just fine, 100% of trials.

Opening using the IP-address doesn't change the picture, so doesn't look like a DNS issue.

Any ideas?

from wordpress err_connection_reset in Chrome from specific country