If something on the page violates a mod_security rule, the server won't let it be saved to disk or written to the database. The modx_security rule set on some hosts changes often and quite often the trigger is something it considers a dangerous word, phrase, or bit of code. This would not show up in the MODX Error log because it's not a MODX or PHP error. I don't think it will show up in the server error log either, but I could be wrong.
One user was unable to save any chunk that contained the word "casino," and rules about bits of PHP, JS, or HTML code (e.g., preg_replace, str_replace, script, base64_decode, etc.) are common.
The bottom line is to ask your hosting service. They should be able to see it in their logs. If they're not willing to help you, it might be time to change hosts. It's unpleasant, but if the host has restrictive mod_security rules and won't lift them, there are bound to be other problems. If you end up switching, this might be helpful:
MODX-Friendly Hosts
