VF31 processing error

Dear Friends,

While processing billing document in VF31 for output media Fax, when we select processing mode as 3 - Error Processing (All)

Or 4 - Error processing (only new message) We are getting message as Errors occurred while processing output.

On checking log we found message as "Cannot be processed: Message has been changed"

Please let us know what might be the reason?

Thanks in advance