CiviMail template errors
I had the same problem. It happened after upgrading CiviCRM. The cause was the html. There were problems/errors with the html template along with too many tables. We used the W3 validator (https://validator.w3.org/). That helped a lot but did not find all the problems. So we just kept testing portions of the html until we found the section that was preventing CiviCRM from saving. Eventually we were able to clean up the html and CiviCRM was able to save it.