I was configuring one of our clients farm which required FBA on different web applications (around 5) and he wanted each web application should have separate user store. That was too tricky since you just need to set up Application property of the membership provider and you are done.
After completion of these settings, all of my applications were having a common exception either in log out or login to the web application. Here was the exception.
System.ArgumentException: Exception of type 'System.ArgumentException' was thrown. Parameter name: encodedValue at Microsoft.SharePoint.Administration.Claims.SPClaimEncodingManager.DecodeClaimFromFormsSuffix(String encodedValue) at Microsoft.SharePoint.Administration.Claims.SPClaimProviderManager.GetProviderUserKey(IClaimsIdentity claimsIdentity, String encodedIdentityClaimSuffix) at Microsoft.SharePoint.Administration.Claims.SPClaimProviderManager.GetProviderUserKey(String encodedIdentityClaimSuffix) at Microsoft.SharePoint.Utilities.SPUtility.GetFullUserKeyFromLoginName(String loginName) at Microsoft.SharePoint.ApplicationRuntime.SPHeaderManager.AddIsapiHeaders(HttpContext context, String encodedUrl, NameValueCollection headers) at Microsoft.SharePoint.ApplicationRuntime.SPRequestModule.PreRequestExecuteAppHandler(Object oSender, EventArgs ea) at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
After checking all my steps right through starting. After getting all the other exeptions cleared the above exception was the only exception that I was getting. That was resolved using "Exception of type 'System.ArgumentException' was thrown" got error when i have signout from webapplication in sharepoint 2010?.
I hope this will help to you if you are getting same error due to a silly custom property of a custom web part for registration!