I developed a Force.Com SIte that display an account in a form. I have a save button. But i receive this error when i try to save :

system.security.NoAccessException

My VF :

<apex:page showheader="false" controller="DemoForceComSite">
    <apex:form id="ficherenseignementform">
        <apex:pageMessages />                     
        <apex:inputHidden value="{!accountId}"/>
        <apex:outputLabel value="Raison sociale " for="RaisonSociale"/>  
        <apex:inputText id="RaisonSociale"  value="{!oAccount.name}" styleClass="inputtext" />
        <div>
             <apex:commandButton action="{!save}" value="Save"  />
             <apex:commandButton action="{!save2}" value="Save2"  />
             <apex:commandButton action="{!cancel}" value="cancel"  />
        </div> 
    </apex:form>
</apex:page>

 My controller :

public class DemoForceComSite{

    public Account        oAccount {get;set;}  
    public String         accountId {get;set;} 
      
    public DemoForceComSite() {        
        
        accountId = '001P000000WlX5O';
        try {
            oAccount = [Select Id, Name From Account where Id= :accountId limit 1];
       } catch (Exception e) {
                // Display error message
            ApexPages.addMessage(new ApexPages.Message(ApexPages.severity.ERROR, 'Impossible de retrouver les informations nécessaires à l\'affichage de la page. Veuillez prendre contact avec votre interlocuteur.'));
            return;
        }
    }

   public PageReference save() {      
        oAccount = [Select Id, Name From Account where Id= :accountId limit 1];
        oAccount.name = 'Test';
        upsert oAccount ;  
        return null;
    }
   public PageReference save2() {    
        upsert oAccount;
        return null;
    }
   public PageReference cancel() {
        return null;
    }
}

Every method fails. Even the cancel method !! The rights are ok on the object.

It seems that the error not occurs when i withdraw all the account fields on the VF !! But i need them !!

Any idea ?