Convert Drupal-6.22 to Mercury/Pressflow - fatal PHP errors

I setup a Linode and ran the Mercury stackscript (ID=353).

Then I imported my /sites/default/files, sites/all/modules, sites/all/themes, sites/all/libraries, and DB from an existing D6.22 site.

Now I just get Varnish 500 error pages, and the logs show fatal PHP errors like this one:

Call to undefined function features_include_defaults() in /var/www/sites/all/modules/custom_formatters/custom_formatters.module on line 281.

But if I substitute my old /modules, and /includes directories and files, the site works.

I wasn't using the custom-formatters module so I just deleted it, but then the same error just moved to the Rules module, which I do use.

Call to undefined function rules_include() in /var/www/sites/all/modules/rules/rules_admin/rules_admin.sets.inc

All module directories and files in sites/all/modules appear to be normal. At first I assumed it was a file ownership/permissions error, but I checked that and set the site perms per usual. It's probably something stupid I'm overlooking.

Seems Pressflow isn't always a drop-in replacement for D6. To complicate matters, Mercury (and it's components like Pressflow) isn't exactly a community effort so there's no place for centralized support.

Any ideas what might be wrong?