What can cause a 503 'fault'?

I have a ModX 2.8.6 install. Suddenly, after not wanting to save a resource (just spinning), the page went blank. Later I just got 503 service unavailable. The server works just fine - tested with a index.html file - so it must be something ‘Moddy’.
I deleted everything in cache folder and can see that nothing is re-created there, so ModX doesn’t even start up.
Technicians on the shared host can not find any ‘flaws’.
The site is https://bossly.eu/

There are cases when MODX returns a 503.

For example in the file index.php when the file core/model/modx/modx.class.php can’t be found:

In your case though, the server returns the message

The server is temporarily busy, try again later!

I can’t find this message anywhere in the MODX source code.
So maybe it’s a different problem.

Yes very strange. The file modx.class.php is there.
I guess I can try backing up everything and do a fresh install.

Or take a look at the server error log to see if something gets logged there.

The “503 Service Unavailable” message seems to be generated by the server (and not MODX). Maybe because a request to MODX generates a fatal error.

Yes, it is the default server 503 file
The only errors there are regarding a missing 503.shtml
Creating one - that is what shows up. Meaning ModX does not even try

That usually means the server is either overloaded, or down for maintenance, but since it works with index.html, that’s unlikely.

I’d take a look at the .htaccess file. If it looks normal, I’d suspect a problem with mod_security, which might have cause the infinite resource save and set a flag somewhere.

Another possibility is a plugin that’s attached to an event that occurs during MOD initialization. You can disable all the plugins in the site_plugins table, then clear the cache manually.

Thank you for the informative answers.
I figured out where the problem was. I inserted four line-break ‘codes’ </br></br></br></br> in a description field on a resource/page and it wouldn’t save - and later, after trying several times, i got the 503. These were inserted as a quick way of making a div higher.
After working on re-creating the site from almost scratch for two days, I packed it up, unpacked the non-functioning files, uploaded ModXx 2.8.6 and ran the setup, quickly removed the line-breaks, saved and voila - site is back alive.
Very weird experience - who would have thought.

I wonder if use the equally valid <br> tag would have worked. I believe it’s preferred now.

This sure sounds like a mod_security issue. It’s a type of measure that tries to prevent injection attacks on websites. It can sometimes misidentify payloads as malicious. To @bobray’s point, the standard HTML <br> tag might not trigger it.

Yes, that is my conclusion as well. I ‘maintained’ my div height by moving the ‘code’ up into the ‘long-title’ - that apparently does not trigger this.