Why can't I save a Send HTML Email action? I have a rule component set up that currently sends an HTML email. I need to change the body of the email to include a new field, but when I do and press Save, it doesn't return to the Rule page, it just goes to the top of the page and loses the changes I've made.
If I overwrite the HTML with a simple piece of text or a small piece of HTML, it works OK.
I'm wondering if there's some validation going on, but there are no error messages displayed or in the logs. I've validated the html through the W3C Validation Service and cleaned it up so there are only a couple of warnings.
My other thought is that maybe there's some limit that I've hit? The HTML is quite long, as it was generated by an online generator.
I can't include an export of the rule as it's too big for this message.
Any help?
I am using Drupal 7.59 and Rules v7.x-2.10; MimeMail v7.x-1.0. Have tried the latest dev version MimeMail too, but made no difference.
Thanks Pierre for your suggestion. I didn't need to try it, but it makes a lot of sense. Wish I'd have thought of it!

In the end (before I saw your suggestion), I rebuilt the email using a different generator tool, which seemed to produce much smaller and cleaner HTML. 
This works OK. It looks like there was either a problem with the HTML or there is some kind of limit on length.