The problem is occurring for 2 reasons I think.

• Someone has a space in the username part of their email address
• Cron tries to email them and is considered invalid email address

The log entry fails to be added because url parameter is blank during cron processing.

Current case is email with username part of their email address.
Email assumingly forum trying to be sent to them via cron.

This is the error generated via the email_to_user function.

Insert into log table failed at Wednesday 24th of August 2011 09:16:04 PM.

 It is possible that your disk is full.

The failed query parameters are:

array (

  'time' => 1314186363,

  'userid' => '3',

  'course' => '1',

  'ip' => '0.0.0.0',

  'module' => 'library',

  'cmid' => 0,

  'action' => 'mailer',

  'url' => NULL,

  'info' => 'ERROR: You must provide at least one recipient email address.',

)