After upgrade, cron 'cannot be processed'

I'm getting this error message when I try to load cron.php: There has been an error. Your transaction cannot be processed.

This is a "new" site just moved over from a different domain on my server, where it was running under another domain's Drupal 6.12 within a multi-site configuration.

I gave the new domain its own 6.13 installation, pointed settings.php to the existing databases, put .htaccess in place, copied over the theme the site needs, and duplicated all the contributed modules from the shared 6.12 installation (then deleted the ones not being used by this site).

Once all this was done I opened the site and it looked perfectly fine except for a database error (about comment signatures) showing on some pages, only for authenticated users. The admin status page had two warnings: one about cron needing to run, and one saying I needed to run update.php. I tried to run cron manually and got the "cannot be processed error." Then I backed up the site and database and ran update.php.

Running update.php fixed the database errors but I still cannot run cron.php.

Any ideas?