Frontend registration not working

Just updated to 1.4.5 and when trying to register a new user on the frontend at /user_register I get the following error…

Server Error
Sorry, something went terribly wrong!

0 - Username "erik_the_viking" already exists, please pick another username
For further details please review your logs/ folder, or enable displaying of errors in your system configuration.

Nothing in the logs and there is no page for the user at /user/accounts. I can register users via the admin backend no problem.