Definition of the membership and rolemanager in child web.config

I know that some people will think that this question is specific to SharePoint, but in fact I have a purely ASP.NET configuration problem...

For our SharePoint internet environment I have build a user management web application which is deployed as a sub web under SharePoint website. Although this application is pretty ?SharePoint? I have built it the way it also works (and can be debugged) outside of SharePoint. This web application just uses all SharePoint stuff via services and pretty independent. So just copying/registering the dlls, aspxs and web.config to the web under SharePoint web is pretty all I do for deployment. However I encounter problems with web.config of my web application. Definition of the membership and rolemanager in application config (child web.config) causes ?An unexpected error has occurred?. When deployed to the web outside of the SharePoint, application works pretty OK. Also removing the membership and rolemanager from child web.config solves the ...