One reason you may receive the following error when editing/saving the email template is because text content contains double hyphens/dashes.

The email template is based on HTML, and double hyphens/dashes "--" are not allowed.

If the text can be edited/changed, remove the -- and simply use a single hyphen/dash -.

If the text is a URL, then replace -- with the encoded value %2D%2D.  For example, somewebsite.com/this--is-the-link would become somewebsite.com/this%2D%2Dis-the-link.