Thanks to the list ...
I received many answers : one solution is to reboot the netapp with floppy, at the menu choose "boot without /etc/rc" then delete the offending quotas file. In my case the error came from an unknown user being assigned a quota.
But ... this problem is already known by netapp which kindly created some time ago a new release of data ontap ( bug number 26472 corrected by 5.3.6R2). I'll wait for my NOW account ...
The filer is now up and waiting for my next error :)
thanks one more time, Jean-Marie