I think this is due to a problem with the regular expression near the end of the core\model\modx\mysql\modchunk.map.inc.php file. It's usually due to a PHP update, which happens regularly at most hosts.
'rule' => '/^(?!\\s)[a-zA-Z0-9\\x2d-\\x2f\\x7f-\\xff_-\\s]+(?!\\s)$/',
The reported fix is to change it to this (the _- about two thirds of the way through is reversed to -_ which should fix it):
'rule' => '/^(?!\\s)[a-zA-Z0-9\\x2d-\\x2f\\x7f-\\xff-_\\s]+(?!\\s)$/',
The upgrade might well fix it. I know there was a pull request on this a while back.
In any event, you should definitely upgrade MODX to at least 2.2.15. The versions you have are not secure and there have been a number of reports of those versions being hacked.
http://modx.com/download/previous-releases/