I have a custom button on a custom object that executes some javascript which calls a web service method and populates parameters to call the EmailAuthor link in SF. In sandbox it is working just fine and the code I cannot see any difference. The problem is when I click the button in production I get the following message:

The value of the "p3_lkid" parameter contains a character that is not allowed or the value exceeds the maximum allowed length

This is the code of the button:

{!REQUIRESCRIPT("/soap/ajax/10.0/connection.js")}
{!REQUIRESCRIPT("/soap/ajax/10.0/apex.js")}

var docId = sforce.apex.execute("SalesDoc_PDFAttachment","getSalesDoc_PDFDocument", {salesDoc_ID:"{!SCRB_SalesOrder__c.Id}",pAttID:"{!SCRB_SalesOrder__c.Active_PDF_Attachment_ID__c}" });

location.replace('/_ui/core/email/author/EmailAuthor?retURL=/{!SCRB_SalesOrder__c.Id}&p2_lkid={!SCRB_SalesOrder__c.Selling_ContactId__c}&rtype=003&p3_lkid={!SCRB_SalesOrder__c.Id}&doc_id='+docId);

I cannot see any errors and when the link is executed the values populated into the url are:

retURL= the id of the custom record

p2_lkid= the contact to send the email to

p3_lkid= the id of the custom record

doc_id= the ID value of the SF document returned by the web service method.

Can anyone spot where this is failing?