Migrated Servers now 500 Error _db_error_page('Access denied f...')
and set server settings to match old server


That process went wrong - ereg() was removed in PHP 7.0, and Drupal 6 couldn't have been running on a version of PHP that high on your old server.

I think PHP 5.3 is about as high as you're likely to be able to get with Drupal 6, you should get your server admin to downgrade to that version ASAP.