Sounds like an infinate redirect/rewrite, which means that the URL is being rewritten, then the new one is matched and is rewritten again, then the new new one is matched and rewritten again, etc etc.
Of course, your host may not support mod_rewrite.
Try preceeding your rewrite rule with a condition that checks whether it’s already been rewritten. For example, the following rule only rewrites the URL if it’s not already been rewritten to /modx/index.php.
RewriteCond %{REQUEST_URI} !^/modx/index.php
RewriteRule ^(.*) /modx/index.php?q=$1 [QSA,PT,L]
Saying that, you’ve not given enough details for me to give a full answer... Are you using the standard modx rules? Do you have friendly URLs on? What are your rewrite rules?