I can't figure out why the 404 page is incorrect?

Question

MODX

Hello.
I stumbled upon a strange thing: there is a Tuts website , made on MODX REVOLUTION, the version of the engine is very respectable, but it still works, albeit with a creak - too many resources are crammed. This site has a resource 404, configured as expected - the id of the corresponding resource is registered in the system settings.
The strange thing is that if the motorespect.com/catalog link is slightly changed, for example, like this: motorespect.com/catalog2 , then the resource 404 is displayed, as expected. But if you change the alias of the subpage, that is, it was like this : motorespect.com/catalog/equipment , enter like this - motorespect.com/catalog/equipment2, then the service page of the server is displayed, also, as I understand it, giving 404 (although it says "503 error").
This is the first time I've encountered this, can you advise which way to look?
PS. Disabled "suspicious" plugins - same result...