503 service unavailable | Error

[noyle]

If your forum is large or you have a large number of active users (logged in ones and guests), the server resource might be the limit. You should check if server's CPU/memory usage continues to spike. If so, it might be the time to upgrade your server.

MyBB should be less a resource eater compared to some other PHP applications (WordPress I'm saying). But if your forum is small and has seldom visitors (including bots/spiders) and it still gives you 503 frequently, it might be something wrong with your plugins.

I think this thread is not involving real security problem, so I'll have it reported for mod to move it to the General Support forum.

Oh, forgot to answer your question:

(...)
I don't know where those files are used.

Some of these items are logged when a stranger is accessing a non-existing resource (page/image/js/etc.). It's OK to have that, but if you mind, just search the Internet for that, like "why my site has so many 404 not found error logs".

File not found [/home/111111/public_html/403.shtml] or other ...503..., ...404... means that your HTTP server is set to serve a special page, whose path is exactly the path logged in your report, to a visitor if this visitor's request has return a 403/503/404/etc. HTTP code, but the actual page by that path doesn't exist.

File not found [/home/111111/public_html/images/Dark1.8/js/dropdownmenu.js], for this one, looks like your theme (maybe named Dark?) is using such file but that file is not uploaded. Otherwise, consider it as a bot visiting.