Just been getting this error. It’s quite common, especially if you’re using Ajax. I got the error for an altogether more ‘schoolboy error’ reason.
I had a textbox control e.g. called ‘customerName’, and two radio buttons controlling whether to show the customer name. These had a ‘GroupName’ of customerName, and this caused the error. You need the GroupName to associate the radio buttons together, but the postback mechanism obviously sees that as the control identifier. Silly boy.
The naming was pretty bad in any case and actually prompted me to take more of a long hard look at the UI to tidy it up so no such conflict would arise.