Sharepoint -> backup -> stsadm.exe = Access denied?

Question

SharePoint

Hey!
We have been using sharepoint on windows server 2008 for a long time and previously everything was backed up with a few lines in the batch file:
cd /d %COMMONPROGRAMFILES%\Microsoft Shared\web server extensions\12\BIN
stsadm.exe -o backup -directory D:\Backups\sharepoint\ backup -backupmethod full
Today I noticed in the logs that backups began to weigh 1kb. I tried to manually make a backup through the command line under the administrator, writes "access denied". On the Internet they write about disabling UAC as a panacea, but it did not help me.
Who faced? Any ideas?