@SERROR after .SetInfo :
COM exception error "SetInfo" ((null) - (null)) [-2147352567/80020009]

code :
$AD=GetObject("...")
For Each $User in $AD
$User.cramBac=$Bac
...
$User.SetInfo
?@SERROR
...

if $Bac is empty or "", it does not work.
Of course, cramBac property exists for the object.