Today I got below error when I was started a new migration from windows plesk 9.5 to Plesk 11.03 and the migration failed
Unable to start Deployer because there is another Deployer run
RDP to remote server and delete all files in below folders
C:\Program Files (x86)\Parallels\Plesk\PMM\msessions C:\Program Files (x86)\Parallels\Plesk\PMM\rsessions C:\Program Files (x86)\Parallels\Plesk\PMM\tasks
After that restart the plesk migration agent.
If the issue still persists then restart the server once and try again.