Kindly double check with your reservation team if they are still able to use the manual contact without the validation now.
As for this validation, it has just been added up on the latest patch which is using in your site. So, they should not be able to go through without the validation now.
We have also notice the issue where no contact inserted earlier and currently checking with tech if we are able to remove the validation. In the mean time, you might want to either insert a dummy contact into the creditor/supplier or set MS_MULTI_CONTACT = N .
Once the INI setting of MS_MULTI_CONTACT set to be N. You will not able to send one mail to multiple contact. If you wish to do so, you will need to send the mail numerous time. Please find the attached [MultiContact.docx] for your reference