Error on calling VF page from another VF page using the same controller. The page you submitted was invalid for your session

Question

I am trying to call a VF page from another VF page which use the same controller.

I am getting the error when i click on the command button which calls the pagereference method and redirects to the next VF page

The page you submitted was invalid for your session. Please refresh your page and try again.

Apex Class

public pageReference SaveNext()
    {
        Savepoint sp = Database.setSavepoint();
        Try
        {
            showErr = false;
            update opp;
            update services;
        }
        catch(exception e)
        {
            showErr = true;
            errMsg = e.getmessage();
            Database.rollback(sp);
            return null;
        }
        PageReference prep = Page.WonPrepCustomer; 
        prep.getParameters().put('id',opp.Id); 
        //return new pageReference('/apex/WonPrepCustomer?id='+opp.Id);
        return prep;

    }

VF page (Commandbutton code)

<apex:commandbutton action="{!SaveNext}" value="Next to Customer Prep" styleClass="slds-button slds-button_brand" rerender="errorParent"  />

Looking through all the other posts regarding this issue. Here are things i already tried

1. Enabled in Session settings

Enable clickjack protection for customer Visualforce pages with standard headers.

Enable clickjack protection for customer Visualforce pages with headers disabled

2. Added Showheader=true on <apex:page/> tag.

3. Use Page. instead of partial url.

4. Issue remains on classic and lightning experience

Is there anything else that i can try?

Answer 1

You should use setRedirect(true) with your PageReference.

Description according to documentation:

SetRedirect() is a method of PageReference class. When a PageReference value is used as a return value for a commandButton or actionFunction method, the setRedirect() value defines whether it is a client-side redirect or a server-side forward. Thus if you use this with true param controller data will be dumped as it will request server meaning everything is reinitialized, using the class constructor if any, all the variables will be dumped etc and if you use the same controller in 2 pages the error was caused by the pre-existing values that were carrying over to the next page since it is using the same controller.