I reply to this thread since i get nearly identical error:

When i start on a debian sarge linux a partimaged (-v shows 0.6.4) which is correctly working for debian2debian images and use it with partimage client from Knoppix 5.0.1EN there's always the user credential error showing up.

I didn't get a solution on 5.0.1 but was able to use 3.9.1 successfully.

Seems to be a heavy bug in knoppix 5.0.1, even it shows the same version 0.6.4 as on the working debian and knoppix...
This is really a poor situation since knoppix 5.0.1 woulb be best prerequisite (improved hw-support to 3.9.1) for backup/restores on any computer here ;-(

Any workaround welcome!