Task scheduler fails after admin user renamed

I renamed user on server from "administrator" to "serveradmin" because of recent worm. For few days everything work great but now recuring windows tasks fails to execute with status:

Logon failure: unknown user name or bad password (0x8007052E)

Also, it is not possible to create new tasks:

Task Scheduler service is not available or the folder does not exist. Verify that the service is running and that the folder exists.

Also, it is not possible to modify settings of existing tasks. It states that Author of tasks is [servername]\Administrator.

Any ideas?

For any tasks you have setup that used the old administrator username you need to change the credentials to use the new username and update the password. This should then resolve the authentication issue for the tasks which cannot run as the username administrator user is no longer in use.