You have verified that the account password is correct and the account is enabled.
And when running tests they fail to run with the following message. "Failed to validate credentials specified for test execution: either the account password was changed, the account was disabled, or the time on the Diagnostic Services computer is not synchronized with domain controller. The test will be disabled to prevent the account from locking. Verify the time synchronization or update your credentials using the Global Settings, and enable the test."
The account currently running the Spotlight on Messaging services cannot be specified as Alternate Credentials.
Alternate Credentials are used in the case that you want to use an account other than the service account to run the health test.
Remove the service account from the list of alternate credentials and in the test settings set the Credentials Settings to "Use default Diagnostic Services Credentials"