This is a separate issue from the intermittent outages experienced this morning. Several of our customers are reporing that non-admin users in Kiln are seeing "Kiln has overheated" error messages on their Kiln On Demand websites as well as 500 errors from the Kiln client. As a workaround, you can temporarily make the users who can't access Kiln site admins and then have them try to log in again.
We're actively investigating this issue and will post an update once it has been resolved.