"Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request."
In that page there were these control: a TreeView and a Placeholder,when the user selects any node then it's controls will be created dynamically into that placeholder..for the first time it's working fine but when (s)he select another node then that issue appears.
He called me to help him with this issue,for me this is the first time I see such an issue,scratch my head then I decided to eliminate the possibilities of this issue one by one,at the development machine it's working perfectly,he published the website at the local IIS and again..it's working perfectly,I took a copy of the website and published it into my laptop but no issues at all,so this is means that it's not an issue in the code.
So there is something missing/wrong in our server [it has Windows Server 2003],we went to the server and checked on the web-config and the configurations on IIS...nothing wrong so far,so I decided to check if the framework 3.5 is installed or not and the answer: it wasn't installed
Of course he assumed that it was installed and there was nothing to tell if it wasn't from the "ASP.Net version" in IIS because frameworks 3.0 and 3.5 will not be listed there [2.0 will be listed there instead],the only way to check if it was installed or not is to search for the framework in this path:[WINDOWS Folder]\Microsoft.NET\Framework or check if it was installed in Add or remove programs.
The obvious solution for his case: We installed Framework 3.5 SP1 into our server,did a restart to the machine and it worked !
If anyone faced the same issue and solved it using the same solution or with a different one please post it here to share experience.