I don't do multi-context sites any more, so I'm not much help here. Are you sure you don't mean [[++assets_url]]? That's what you need for images, CSS, and JS files. It's easy to get them confused.
Check the MODX_ASSETS_PATH and MODX_ASSETS_URL settings in core/config/config.inc.php - that's usually what's used for any access to the assets area. If that doesn't work, I'd go with PaulP's suggestion.
@BobRay, thanks so much for you time. I'll try PaulP's method. How do you handle multiple languages then?
@paulP, thanks for your help. I've tried it but no luck yet, maybe the .htaccess needs to sync. But this piece of code can rewrite my html path?
TBH, I've never done a multi-language site.
Make sure PaulP's code is above any other rewrite rules, but after the RewriteEngine on and RewriteBase line, which should be:
I'm glad we could help. Paulp deserves most of the credit.