Cube MOLAP to ROLAP, what do I do wrong ?

We changed a working MOLAP cube to ROLAP.

We now have a strange issue :

Working : Displaying one dimension attribute and one mesure.

Not Working : Displaying more than one attribute and one mesure.

I always get the error : "Un doublon de clé d'attribut a été trouvé lors du traitement de : Table 'MainQuery', colonne 'myColomn', valeur '2011'. L'attribut est 'myColomn'" (In French)

What do I do wrong ?

Thanks for your help.