I installed an English Windows with Converter standalone, I've run a Convert Machine Job with Final synchro, Guest OS customization and reconfiguration.
It failed with the same error at 97 %.
Then I've run a Configure Machine job, and no error occured.
With this task, there is no agent (and no problem with french unicode...)
Thanks for your useful help, but I don't understand why the agent is not programmed to use the same temp path for all OS languages (C:\Windows\TEMP\vmware-temp\vmware-SYSTEM\deploypkgxxxxxxxx\deployPkg.cab)