Basic authentication fails randomly

Basic authentication fails intermittently. I created an HttpHandler with the intent of handling basic authentication myself, but it seems Sitefinity catches requests with the header and authenticates the request against its provider. This is fine, I set up the user so it passes, but randomly it fails. I have no why it fails, we just get an exception. I'd be happy with either of the two solutions

1. Get the authentication to be consistent and always accept the request (when the credentials are correct of course)
2. Not have Sitefinity process the authentication header and let me authenticate myself.

Exception

Source: Telerik.Sitefinity
Message: Failed to authenticate. Reason: Unknown
   at Telerik.Sitefinity.Security.SecurityManager.IsBasicAuthenticaton(HttpContextBase context)
   at Telerik.Sitefinity.Security.SecurityManager.AuthenticateRequest(HttpContextBase context)
   at Telerik.Sitefinity.Web.SitefinityHttpModule.AppContext_AuthenticateRequest(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)