|Knowledge Article Number||000229821|
This is an intentional change which has been made by Salesforce in Winter'16 release to remove security vulnerability.
Staring Winter'16, you will not be able to use the Custom Buttons to call standard URL for Approval Process Submission (which is "/p/process/Submit?... ").
When you use this, you will get an error : "We need to confirm that your request is valid. Please repeat your last action"
This is working as designed.
|Resolution||In order to make the custom buttons for Approval Process submission work, you will need to use Apex or REST API Winter'16 onwards.|