This is all I got in the log:
<?xml version="1.0" encoding="UTF-8"?>
<execution-result status="error" log-location="/opt/psa/PMM/sessions/2012-11-14-040202.367/migration.result">
<message id="b459e712-0ecb-4ae9-856a-3f0f192e46ff" severity="error" code="fatal">
<description>Runtime error</description>
</message>
<message id="ad394432-40e4-46f8-8f5e-b2eebb2df5e4" severity="error" code="ExportFailed">
<description>The dump have been made successfully but can not be imported to FTP due to errors.
The dump 'backup_info_1211140402.xml' have been leaved at repository '/opt/psa/PMM/tmp/backup2m6uGW'. Please export it manually</description>
</message>
<message id="7b784db7-94c5-4f1f-86be-26589b22e346" severity="error" code="FtpError">
<description>Can't upload file '/opt/psa/PMM/tmp/backup2m6uGW/backup_info_1211140402.xml.tar' to ftp: Error during write to file
Transfer aborted
596.809 seconds (measured here), 41.13 Mbytes per second
?
</description>
</message>
</execution-result>
There's no info why it didn't delete excess backups.