This message normally mean that you are trying to restore your data using
the Ver2 client after you used the Ver3 once.
What happens is the format of the version 2 client's data stored on the
server is different from the way a Ver3 client's data is stored.
If you upgrade you client to version 3 and run a backup, the data that
was backed up using the Ver2 client is converted to the Ver3 format.
If you then later try and access the data using your Ver2 client you will
get the message "format unknown".

Make sure you are using the Ver3 client for your restore.

Hope this helps

Regards
Christo Heuer
ABSA Group
Johannesburg
SOUTH AFRICA