I had a fully functional custom filter web part built. I used the example in the developer guide under Development > Web parts > Data source web parts: Developing custom filters

Over the weekend, I installed a hotfix and am now at v7.0.49.

Now the custom filter is not functioning properly, and I cannot check out the page that utilizes the filter. Anytime I attempt to check out the page, I get the following error:

here was an error processing the page. The error can be caused by the configuration of some component on the master page. Check the master page configuration or see event log for more details. Original message: 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.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Exception: There was an error processing the page. The error can be caused by the configuration of some component on the master page. Check the master page configuration or see event log for more details. Original message: 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.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:


[Exception: There was an error processing the page. The error can be caused by the configuration of some component on the master page. Check the master page configuration or see event log for more details. Original message: 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.]
CMS.UIControls.PortalPage.OnError(EventArgs e) +408
System.Web.UI.Page.HandleError(Exception e) +164
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +12573451
System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +12572973
System.Web.UI.Page.ProcessRequest() +119
System.Web.UI.Page.ProcessRequest(HttpContext context) +99
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +913
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +165

Does anyone have any ideas as to where the problem is?