Synchronize users to contacts fails

I've got CiviCRM 1.9.12432 and Drupal 5.6 on a test site. I've been able to create users and contacts which tells me the database connection has been configured correctly. However, when I go to Synchronize Users to Contacts, I get the following error message:

-------------------------------------------------------------------
unrecoverable error
Sorry. A non-recoverable error has occurred.

Return to home page.

Error Details:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -24
[message] => DB Error: connect failed
[mode] => 16
[debug_info] => [DB Error: connect failed]
[type] => DB_Error
[user_info] => [DB Error: connect failed]
[to_string] => [db_error: message="DB Error: connect failed" code=-24 mode=callback callback=CRM_Core_Error::handle prefix="" info=" [DB Error: connect failed]"]
)