You are backing up an entire machine with the Application-aware backup option enabled. You have selected SQL, Exchange, or both.
After some time, you change the password for the user that has been specified during activation of the feature.
The backup starts failing with the following error message:
Logging on with the specified credentials is impossible, probably because the account does not exist or the password is invalid.
In an encrypted form, Acronis Agent stores credentials to the specified account and uses them during a backup to gain access to the databases. Full text of the error message shows the name of the affected user. If you have change the user or its password, it needs to be updated in the agent as well: re-select the necessary applications in the options of the backup plan and provide new credentials.