Hi all!   I've been fighting with DN 5.1's 'feature' that makes sure you don't goof up your login page.  Have you seen the following error:

A critical error has occurred.
You have been redirected to this default Login Page because the Login Page set for this website does not contain a valid DotNetNuke Account Login module, or the permissions have not been set correctly.

If so, you either REALLY have messed up your login settings, or you are trying to use a custom login module.  Some suggest putting a login module on the page and setting the visibility to admin only.  If you don't find this solution appealing, I have an alternative.

1.  BACKUP YOUR DNN DATABASE!  You don't want to lock yourself out accidentally!

2.  Go to your sql manager and locate the ModuleDefinitions table

3.  Locate the record where the 'friendly name' = 'account login'

4.  Rename this to something else, perhaps Account Login DNN

5.  Find the friendly name of your custom login module (in my case alldnn.login) and rename it EXACTLY to "Account Login"

6.  DNN now thinks your custom login module is the built in DNN module, so your error message goes away!

WARNING!  I'm imagining that the DNN folks are working on a fix to this now.  Remember your original settings in case you need to change them back for compatibility with future releases!

Enjoy!

Perhaps I should specify when this error could occur.

1.  You have created a custom login page and have set up that page in the site settings as the default login page.

2.  You have a 3rd party login module of some kind in substitution of the default dnn module.

Here is an example of this error showing up previously in the forums:  http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/111/threadid/282084/scope/posts/threadpage/3/Default.aspx

I solved this issue to my satisfaction on my end and wanted to share my solution.  I didn't particularly like the workaround outlined in the above link.