SharePoint 2010 Mixed Mode Custom SignIn ArgumentException

Question

I'm using an ASP.Net LoginView to sign users into my web site. The authentication process runs through successfully, but when the user is redirected back to the designation url provider, I get the nasty ArgumentException "encodedValue" error and can do nothing until I close the browser and re-open it.

I've had a look in ULS and it only returns the same error, nothing specific, and I'm running out of ideas as to where to look next for this the cause of this error?

[Edit:]

The stack trace from both ULS and Event Viewer (both shows the same thing):

Stack trace:    at Microsoft.SharePoint.Administration.Claims.SPClaimEncodingManager.DecodeClaimFromFormsSuffix(String encodedValue)
   at Microsoft.SharePoint.Administration.Claims.SPClaimProviderManager.GetProviderUserKey(String encodedSuffix)
   at Microsoft.SharePoint.SPGlobal.CreateSPRequestAndSetIdentity(SPSite site, String name, Boolean bNotGlobalAdminCode, String strUrl, Boolean bNotAddToContext, Byte[] UserToken, String userName, Boolean bIgnoreTokenTimeout, Boolean bAsAnonymous)
   at Microsoft.SharePoint.SPWeb.InitializeSPRequest()
   at Microsoft.SharePoint.WebControls.SPControl.EnsureSPWebRequest(SPWeb web)
   at Microsoft.SharePoint.WebControls.SPControl.SPWebEnsureSPControl(HttpContext context)
   at Microsoft.SharePoint.ApplicationRuntime.SPRequestModule.GetContextWeb(HttpContext context)
   at Microsoft.SharePoint.ApplicationRuntime.SPRequestModule.PostResolveRequestCacheHandler(Object oSender, EventArgs ea)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Answer 1

Have you done the required setup to support FBA in SharePoint 2010, most notably updating the web.config files for 1) the web application using FBA, 2) SharePoint central administration, and 3) SharePoint's SecurityTokenService? I have seen that error in the past when all 3 of the aforementioned web.config files were not done properly. There is a fairly comprehensive walk-through of this process here.