Your script isn't getting the usual 503 error we experience here where it's caused by the server being overloaded; loading the page actually shows this error message in the body of the page: Could not load MODX config file.
The script then throws its own 503 error to call attention to the error; you need to look into what's causing it to be unable to load the MODX config file. Unfortunately most of what I'm finding is in russian, and attempts to translate it are returning only a peculiar mish-mash of words that make less sense than alphabet soup