This line:
WebSecurity.InitializeDatabaseConnection(connectionStringName: \"DefaultConnection\", userTableName: \"UserProfile\", userIdColumn: \"UserID\", us
Just to add another possibility (which I encountered) - which might be the case if you're developing/maintaining an Azure WebApp, using a connection string saved in Azure's Application Settings.
Beside each connection string in the Application Settings is a dropdown for the connection string type - it's very easy to forget to set this to 'Custom' for Entity Framework values and leave it at the default (SQL Database) - which also causes the above error.