Sometimes when there are a large number of input fields on a Asp.Net form, and doing postback, I would get this error:
“Error – Operation is not valid due to the current state of the object”

As it turned out there is a restriction on how many fields can be posted back to server and the large grid that contains a large number of textbox fields probably was the culprit. Google search returned some postings that all point to the configure setting

<appSettings>
<add key=”aspnet:MaxHttpCollectionKeys” value=”2000″ />
</appSettings>

 

After I stuck this line into the appSettings, the problem went away.