>Yesterday, I thought I resolved the issue of "Unable to validate data" by changing the Web.config file to turn off the enableViewStateMac setting. But, today, I got a situation where the following error was returned:
>
>"Unable to read beyond the end of the stream"
>
>I am pretty sure this is related. Because, I never received that error before. This is the stack trace:

I can't say what is causing the invalid viewstate (there are some things your code can do which can cause this problem like adding controls dynamically), and some things out of your control. But I'd say the error is that the posted back viewstate was corrupt (missing data, data truncated, etc.) and when it attempted to deserialize the information in the viewstate back into the statebag it failed because some of the data it was expecting was missing.