HTML content save issue in posts table of WordPress website

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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.
Immediately! However, this has been resolved! It was due to server firewall. It was blocking update due to some content(HTML included) being flagged as executable or malicious. Thanks
– Prajwol
6 hours ago
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"connection reset" often occurs after several minutes -- how long in your case?
– Rick James
20 hours ago