We're in pretty weird territory, especially with Plesk involved (which I know nothing about other than that other MODX users using it have run into trouble).
This could be relevant:
https://forums.modx.com/index.php?topic=66702.0
If that doesn't do it, it might be time to start looking at the permissions and ownership of the directories.
Another possibility is that it's mod_security having a problem with a string in the URL or the $_REQUEST.
Huzzah!
I guess that .htaccess file is magic. there's not much in there, but whatever it is fixed this issue.
Thanks for the link, and I'm going to go thank that poster also.
Cheers, and thanks again Bob!
I'm glad that worked. Thanks for reporting back.
