We are having a similar issue in Odoo v9. Modifying the templates without being aware of the tag requirements can result in this error. However, editing the mail template again and using the 'code' view in the content editor to reinstate the default system template did not fix it. We compared this template to a fresh install and there was no difference.

In the end, the only thing that worked was finding the corresponding entry in the PostgreSQL database and reverting that back to the system default. Suddenly it works fine.

It certainly seems from here that the Odoo editor features for mail templates will break the formatting no matter how you submit them (wysiwyg or code view). I think it should be considered a bug.

Hi I am on version 15 of odoo and the way to use IF conditions is with
And inside the t you write what you want to do

Updating a bit for this original question for Odoo 13. The issue is still present and can easily cause total e-mail template rendering failing if one evaluation fails. We ran into this when modifying Spanish language version of the offer e-mail template. A typical use case is to check if a field has content or not before printing it out to the e-mail template as fields without any content set to them evaluate as text "False".

In our case we wanted to print tittle of the recipient if he has one ("Dear doctor Stephen Strange" if Stephen Strange has tittle defined). The field is object.partner_id.title.name and to check if it is filled, the code is following:

So the whole block becomes:

Oh, through years I've stumbled on this issue again. Thank for sharing this remedy!

There is an issue there the %if &endif is not working, i don't know why, the way i solved was this:

I did it in the mail template for sale.order:

     Order reference: ${(object.origin or 'No Origin Document'}

Hope this works in your mail template.

I have almost the same problem the template is just not fully parsed, I have edited the default invoice and sales order templates and need to be able to restore these back to the original ones, on a development server I was able to do this by removing the templates and update the module account_voucher, but on my production server I have always 504 Gateway Time-out (nginx)

have tried another way by exporting these both the templates and the Dutch (BE)/ Nederlands (BE) translations on a new server installation and import it on the production server but seems not to work. 

It should work if the fields are specifed are in the object you selected.
see the below sample applied to sale.order object which %if %endif works perfect.

  Order number: ${object.name}

  Order total: ${object.amount_total} ${object.pricelist_id.currency_id.name}

  Order date: ${object.date_order}
% if object.origin:   Order reference: ${object.origin}
% endif % if object.client_order_ref:   Your reference: ${object.client_order_ref}
% endif % if object.user_id:   Your contact: ${object.user_id.name} % endif