Membership Registration Error

I'm getting an Unknown Error when trying to register a user. Looking at the logs, the error is:

" Failed to open the file '14\Resources\FBAPackFeatures.en-US.resx'... "

Looking through the directory, it indeed doesn't exist, there's a plain FBAPackMembershipRequestWebPart, zh-CN and a zh-HK, but not an I missing a file or using an old version of FBA? I downloaded it last week...

You're not missing a file. Sharepoint records this error when it doesn't find a resource file for your exact locale, and uses the default resource file. There is no en-US resource file as the default resource is english.

Unfortunately it also throws you off the track of what is causing the unknown error message. There should be another exception raised in the log file. Most likely there's an issue with your SharePoint outgoing email configuration.