Hi Igor,
I had the same issue. I didn't realize because my script is made to send me a mail once the backup is finished, but I didn't make it to monitor the pleskbackup script status.
Since 25th of May on one Plesk 11 server, 28th of May on the other one, the backup was frozen. My guess is that it is related to the latest plesk update.
Tell me the scripts you need, and I will provide you the scripts for both servers, in order for you and your team to troubleshoot. This might be a one time bug during update deployement, or a bug in the deployement tool...
Cheers
Akim