We recently came across an error that I could not find in Veeam’s support site (though it did return several hundred, seemingly unrelated entries), the error was
Error code: 0x80040154 Failed to create instance of DOMDocument
This caused our backups to fail. I sent a tweet to @Veeam to see if they had anymore and in typical awesome fashion they responded within minutes. Right now there is a private hotfix so you will need to contact support, once I get to review the cause I will let you know. As a band-aid, restarting the Veeam Backup Proxy Service seems to clear this.