Sharepoint backups fail - The logon account that was provided does not have valid credentials

I'm backuping a Sharepoint 2010 Infra with Backup Exec 2010 R3.

Backups were running well for a while, but since 3 weeks, backups fail with error:

Final error: 0xe0009b61 - The logon account that was provided does not have valid credentials. Ensure that the user name and password are correct, and then try again.
Final error category: Security Errors

Nothing has changed in our SP farm or backup server.

When I try to browse the SP farm in the selection list, I have an "None of the Web servers for the specified server farm could be accessed" error as mentionned here: http://www.symantec.com/business/support/index?page=content&id=TECH134986

I followed the previous article without success.

Followed also related articles without success:

http://www.symantec.com/business/support/index?page=content&id=TECH129044&actp=search&viewlocale=en_US&searchid=1391774393031

https://www-secure.symantec.com/connect/forums/adding-sharepoint-farm-none-web-servers-specified-server-farm-could-be-accessed-error-e000fef

I checked persmissions in SP, SQL, Windows, all is fine.