Help With Technical Definitions

Normally, you don’t need too scare with the 404 errors as long as your website is still running correctly.

Basically, 404 error means there is an attempt for requesting a file (or resource) that is not exist on your website.

Why it happened? Many possibilities, it can be:
– You (accidentally) deleted a file or link
– A plugin that is not wrote properly (its author’s fault)
– Hacker attempt

You can see what files or resources on the 404 log. If you deleted it, you may put it back so the problem will be solved.

But if you see the request is linked with a plugin name then you may inform the plugin author hope it will be fix on its next release. This below is example of error linked to a plugin:
/wp-content/plugins/plugin-name/icon.png

Sometimes, it is hacker or spamming attempts. Like these below:
/signup
/register.php
/wp-content/themes/Modest/thumb.php

OK, thanks for this info.
One of the error messages is for a theme that isn’t even installed on the server where the main site is, or for images that I recently uploaded.
That’s why it’s kind of confusing.
Kevin

If the message contains a theme’s name that you never installed, it usually is a hacker attempt, for example:
/wp-content/themes/Modest/thumb.php

The sample above is one of my 404 error. The hacker was trying to access a file called thumb.php, which he managed to know that the file have security vulnerability so he may use it for hacking my website. Luckily, it don’t exist, I even never use the theme.

If the error message is one of the theme you recently removed from your website, it can be the web crawlers (google, bing, etc) visiting your website to check your website to find if there any changes have been made. And it causes a 404 error if you just removed the theme. Usually after some period of them, they (the crawler) won’t back to check the file.

If you have too many 404 errors, it can be an indications that you’ve been targeted by hackers. As it happened on me, they came back everyday, scanning my website searching for security vulnerability, which triggers a lot of 404 errors everyday. If you think you’re under attack, you should enforce your website security by installing some security plugins.

It’s is interesting to learn who they are that causing the 404 errors. You can copy/paste the IP and put it on:
https://whatismyipaddress.com/ip-lookup
https://www.projecthoneypot.org/search_ip.php

I saw from your previous post and know that you just moved your website to a new webhost.

Not fully success moving website may cause some links aren’t properly recorded on new website, so it causes 404 errors.

For better success of moving a website, thing you may do are:
– Disable all the plugins (except the moving plugin) before moving
– Empty the caching before moving if you use caching plugin
– After moving, resave the permalink (menu > Setting > Permalinks)
– Enable all the plugins after moving and save all the plugins’ settings

I frequently moving my websites from one to another webhost. Ever fail, but I have better luck using Duplicator plugin.