I'm having the same error converting 8.6 to 9.5. Converted the .psa backup using pre9-backup-convert and Backup Manager shows "This is not a valid backup file".
Using pleskrestore I get:
Backup file has wrong format.
Dump has critical errors.
The log file contains:
3740: 2010-08-02 00:36:10,046 DEBUG --> <pmmcli.CheckDumpAction object at 0x01A6D310>
3740: 2010-08-02 00:36:10,046 INFO stdin: <dump-specification><dumps-storage-credentials storage-type="local"><root-dir>C:\Program Files\Parallels\Plesk\backup</root-dir></dumps-storage-credentials><name-of-info-xml-file>converted_info_1008020006.xml</name-of-info-xml-file></dump-specification>
3740: 2010-08-02 00:36:10,051 INFO Packet succesfully validated.
3740: 2010-08-02 00:36:10,052 DEBUG <pmmcli.ActionRunner object at 0x01A6D5B0>: doActivity
3740: 2010-08-02 00:36:10,055 INFO Executing <subprocess[3468] 'C:\\Program Files\\Parallels\\Plesk\\admin\\bin\\pmm-ras --get-dump-info --dump-storage=C:\\Program Files\\Parallels\\Plesk\\backup\\ --dump-file-specification=converted_info_1008020006.xml --check-sign --session-path=C:\\Program Files\\Parallels\\Plesk\\PMM\\logs'>
3740: 2010-08-02 00:36:10,082 INFO Execution of <subprocess[3468] 'C:\\Program Files\\Parallels\\Plesk\\admin\\bin\\pmm-ras --get-dump-info --dump-storage=C:\\Program Files\\Parallels\\Plesk\\backup\\ --dump-file-specification=converted_info_1008020006.xml --check-sign --session-path=C:\\Program Files\\Parallels\\Plesk\\PMM\\logs'> finished successfully.
3740: 2010-08-02 00:36:10,082 DEBUG <pmmcli.CheckDumpAction object at 0x01A6D310>: response
3740: 2010-08-02 00:36:10,084 INFO Outgoing packet:
<?xml version="1.0" encoding="UTF-8"?>
<response>
<errcode>0</errcode>
<data>
<check-dump-result>
<dump-status dump-status="WRONG-FORMAT">
</dump-status>
</check-dump-result>
</data>
</response>