We have a custom app set up on our system with the customer objects Training Events and Enrolments.  An enrolment links an Opportunity to a Contact and the training event and appears as a related list on the training event.  I have created a few VF pages with Enrolment as the standard controller which are accessed via a custom button on the related list on Training Event page. 

The idea is that relevant enrolments are selected and when the VF page is access they can all be edited in one go and the changes saved.  IN the last few weeks I am continually receiving the following error message when I click save:

This has only started happening recently (around the time Spring 12 was released to our Sandbox) and SFDC seem to be unable to support as is it a developer issue.  The code has not been altered at all and still works perfectly in production.  Does anyone know if there could be something in Spring 12 whcih is causing this?

The basic code for the pages is as follows:

<apex:page standardController="Enrollment__c" recordSetVar="Enrollments" sidebar="false">
    <apex:sectionHeader title="Email/Print Certificates"/>
        <apex:form >
          <apex:pageBlock >
             <apex:pageMessages />
                <apex:pageBlockButtons >
                    <apex:commandButton value="Save/Send Email" action="{!save}" />
                    <apex:commandButton value="Cancel" action="{!cancel}" />
                </apex:pageBlockButtons>
             <apex:pageBlockSection title="Selected Enrollments" columns="1" collapsible="False">
                    <apex:pageBlockTable value="{!selected}" var="Enrollment">
                       <apex:column value="{!Enrollment.name}"/>
                       <apex:column value="{!Enrollment.Delegate_name__c}"/>
                       <apex:column value="{!Enrollment.Booking_status__c}"/>
                       <apex:column value="{!Enrollment.Invoice_status__c}"/>
                       <apex:column value="{!Enrollment.Exam_results__c}"/>
                       <apex:column value="{!Enrollment.Exam_percentage__c}"/>
                       <apex:column value="{!Enrollment.Hold_Certificate__c}"/>
                       <apex:column value="{!Enrollment.Certificate_status__c}"/>
                       <apex:column headerValue="Review PDF/Open to print">
                       <apex:outputlink target="_blank" value="https://eu1.salesforce.com/apex/ReviewPDFcert?id={!Enrollment.id}">review pdf</apex:outputlink> <br/>
                       <apex:outputlink target="_blank" value="https://eu1.salesforce.com/apex/ReviewPDFcertnoback?id={!Enrollment.id}">review pdf no background</apex:outputlink><br/>         
                       <apex:outputlink rendered="{!IF((CONTAINS($Profile.Name, "System")||(CONTAINS($Profile.Name, "Japan"))),TRUE, FALSE)}"   target="_blank" value="https://eu1.salesforce.com/apex/ReviewPDFcert_Japan?id={!Enrollment.id}">Japan certificate (except IRCA)</apex:outputlink><br/>
                       <apex:outputlink target="_blank" value="https://eu1.salesforce.com/apex/coverletter?id={!Enrollment.id}">coverletter</apex:outputlink></apex:column>
                       <apex:column headerValue="Cert Printed?"><apex:inputField value="{!Enrollment.Certificate_Printed__c}"/></apex:column>
                       <apex:column headerValue="Select PDF Certificates to Email"><apex:inputField value="{!Enrollment.Email_PDF_Certificate__c}"/></apex:column>
                   </apex:pageBlockTable>
              </apex:pageBlockSection>                        
        </apex:pageBlock>
    </apex:form>    
</apex:page>

Many Thanks for any help anyone can offer on this - I am new to development.