The report server has encountered a configuration error. Logon failed for the unattended execution account. (rsServerConfigurationError)

Complete Error:

The report server has encountered a configuration error. Logon failed for the unattended execution account. (rsServerConfigurationError)
 Log on failed. Ensure the user name and password are correct. (rsLogonFailed)

 Logon failure: the specified account password has expired.

Solution:

Go to Start > Reporting Service Configuration Manager > open tab Execution Account

Now enter the correct credentials.

Then try running the report again.The error must be gone.