Even registration emails not going through - registration produces error
This is a known, documented bug that does not seem to have a solution.  Please see ref: https://issues.civicrm.org/jira/browse/CRM-17161

I am happy to help a civicrm wizard debug this :)

Also noted here:

CiviEvent - Error when registering: Could not find valid value for id

Even registration emails not going through - registration produces error

Why is the event registration &quot;Thank You&quot; page generating an error?