Hi Leslie,
See this thread for the background and a workaround for this issue, it is a change in behavior introduced in an ASP.NET security update.
Basically by default only up to 1000 form elements are allowed on postback, the workaround allows you to override that setting to allow more, but it is desireable to not override by too much.
For the Shared Files feature all the buttons and image buttons in the grid as well as lots of hidden viewstate fields for the grid is what adds up to cause this error.
Probably the best solution is to organize the files into folders with < 471 files so that the amount of form fields generated by the grid is within limits. But see the workaround mentioned in the linked thread if you want to increase that number.
Hope that helps,
Joe