2011-04-14 13:46:51.35 Logon Error: 18456, Severity: 14, State: 11.
2011-04-14 13:46:51.35 Logon Login failed for user 'SANDBOX\myadministrator'. Reason: Token-based server access validation failed with an infrastructure error. Check for previous errors. [CLIENT: <local machine>]
A restart of the database service did not help. Neither did a "Run as administrator" to circumvent the UAC.
A discussion on SQL Server Central showed me the way.
I logged off all computers in the domain to refresh my security token, and then a login could be done - also without "Run as administrator"
SQL Server Central: "Login failed for user with token-based server access validation error"