This solved the problem (after restarting ofc). As I don't know ruby at all, I really can't see if this is a issue in the extension or in my configuration. I just thought to mention the solution here - probably helps others or (if correct) could be included sometime.
I think this might be a issue regarding Redmine 1.4 and not my configuration.
I switched to Redmine 1.4.x and new users couldn't autoregister anymore (Error 500).
With some googling I found http://stackoverflow.com/questions/6877233/actionviewtemplateerror-no-route-matches-passing-old-style-parameters and edited vendor\plugins\redmine_http_auth\config\routes.rb to add the line
This solved the problem (after restarting ofc). As I don't know ruby at all, I really can't see if this is a issue in the extension or in my configuration. I just thought to mention the solution here - probably helps others or (if correct) could be included sometime.