Error 4005 Forms authentication failed - ticket supplied has expired

Question

I'm running a website using ASP.NET 2.0. Every now and then (10+ times per day on 100+ users daily) I receive this error: Forms authentication failed - ticket supplied has expired.

Here's my web.config snippet:

<authentication mode="Forms">
    <forms name=".CLLSAUTH" loginUrl="login.aspx" protection="All" path="/" timeout="60" />
</authentication>

I've looked at several solutions, someone mentioned the session timeout, but it's also 60 minutes in my config. Two more things, I'm not running a webfarm, and the app is not being recycled around the time the error occurs.

Any clues?