HTML content save issue in posts table of WordPress website

Whenever I try to save/previw/publish a page/post in WordPress, the connection was reset issue appears. This happens only with content that includes HTML code in it. Otherwise works fine with plain text though.
At first, I thought it was WP issue but content update failed even when I tried directly from phpmyadmin as well as update query via terminal.

I've tried collation like utf8_unicode_ci and utf8mb4_unicode_ci for that column. But no luck.

There hasn't been any modification in server after the site has been deployed. It worked totally fine until last week. I'm kind of sure it's not WP issue.

Here's the server info if it helps

Web server

Database server

phpmyadmin

I don't see any hint in webserver log or mariadb log.

What do you guys suggest in this case? Please lemme know if anything is not clear or need additional information about issue.

Update:

It was being blocked by server's firewall.

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