claims enabled webapp - No windows identity for domain\username

I have a small SharePoint 2010 farm (2FE, 2 APP) and enabled claims authentication on the webapp. User logon with windows username and password. Currently there is no use of claims besides enabling it on webapp-level, but will soon in the future. Everything looks fine, but on developer dashboard and ULS logs I see from time to time following error: Claims Authentication Unexpected g220 No windows identity for domain\username Where domain\username is the currently logged on user. Pages access etc work just fine, so users don't see this error. Anybody know why this error occurs? Anybody have a fix for this?