Hi,

It seems there is something wrong in your email template, did you have made any changes in email templates recently ? 

The issue is with email template, which is getting failed on rendering. You can do as follows:

* activate developer mode
* navigate to settings -> technical -> email -> email templates
* open "Shipping: Send by Email" or any template related to transfer (stock.picking) model
* click on "Reset Template" button.(keep a copy of template if needed, ie, if you have some changes done from ui in template)
* try validating the transfer

Thanks