Thank you for the clarification.
Can you please try the following steps.
1) Revert the web.config to its original value.
2) Make your class inherit from OpenAccessMembershipProvider
3) Login to Sitefinity back-end and go to Administration -> Settings -> Security -> Membership Providers and register your custom membership provider.
4) You should at least specify an applicationName in its parameters collection. For example CustomProvider/
Note that since you are inheriting from the built in provider you can configure the same parameters as in the default ones.
5) Make sure that the Enabled check box is selected for your provider and go to the Users section. From there create a new user in your custom provider and assign it to the administrators role.
6) Log out and on the log in screen select your custom provider from the provider dropdown and try to authenticate with the new user.
All the best,
the Telerik team
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