I encountered the above error in quite a weird way. I replaced a fromview control (TextBox) with a DropDownList and an SQL DataSource and began to get this error in a different place in the application, not related at this control at all. It turned out this is one of those weird ASP.NET errors that are totally out of context. The problem was indeed the datasource control whose connection string was wrong but it pointed to something entirely different.
/* this line was the problem */
In the above I added connection to my sqldatasource using Properties while editing in .ASPX code (not form designer view). The problem was the connection string was built wrong, although it was done by ASP.NET itself.
Fixing the connection string as above fixed the problem.