Error adding Membership Request webpart to page

I have a SSL site using mixed authentication, both windows and FBA is working fine. The FBA pages under site settings all work fine. I created a new page and added the Membership Request webpart and immediately got an error. This was in the logfile: System.Web.HttpException: Could not find the specified membership provider.

I was able to add the Change Password and Password Recovery web parts, however using them generates NullReferenceExceptions.

Any idea what could be going on here?

It sounds like there's a problem with your membership provider configuration (Unfortunately, yes, there can still be configuration issues even if you can login). The web parts query SharePoint for the name of the membership/role providers specified under 'Authentication Providers' for your web application.  It then attempts to access the provider.  So you need to make sure that the membership/role provider names are specified for the zone you are authenticated to. You also need to ensure that those providers are configured and accessible for that web application.  Probably the easiest way to check that is to open up IIS, select the web application you are authenticating to and open the Providers screen and verify that both the role and membership providers you specified are there.

Thanks, you were right on. Somehow the Providers dissapeared from my web application site but were still there for both Central Admin site and Secure Store service. I added them back and the web part is working now. Strange that FBA continued to work even with those providers missing from the web application...

Glad to hear you got it working. I believe it uses the membership providers specified in the Secure Store service to authenticate - allowing you to login even if the web application isn't completely set up correctly.