If you get event ID 18456 with Source MSSQLSERVER in your application and SQL logs with the verbiage of "Login failed for user 'domain\service.account.you.use'. Reason: Token-based server access validation failed with an infrastructure error. Check for previous errors" somebody may have messed with your "CONNECT SQL" permissions.Everybody who reads this blog knows that I'm not a SQL guy, but a lot of advice on the internet suggests that this is UAC related so some of you Platform and AD folks might get asked about the error. Especially considering that the error suggests an issue with tokens and authentication, rather than permissions.Though this isn't the only potential root cause, a deny or revoke or lack of grant can cause this. Especially on 2008 R2. In 2012 a lot of rules changed in regards to grouping but in 08R2 the loss of this would potentially cause various outages.In our case what was happening was the configuration server was no longer accessible by the various SQL servers and this was appearing on that server every minute.Try this:select * from sys.server_permissionsandselect * from sys.server_principalsThis may turn up that builtin\administrators or the service account (or some other important security principle) has a specific "deny" on "connect sql" or... revoke, or even a simple lack of grant.