We’ve built quite a few websites for our clients over the past few years and we exclusively use X. Some clients have waited to make any updates to the theme simply because they were comfortable with the UI from the older X but things started acting funny for some clients when their host upgraded to PHP 7+ so we recommended a theme update.
We found that most of the sites broke when updating X from the dashboard when there was big gap in the versions - even when doing a manual update using FTP. Trying to restore from hosted backups would get the site back up except the WP-Admin page would be broken. We found Cornerstone to be the culprit, as soon as the plugin was removed we were able to access the login and dashboard but trying to reinstall either the old or new version of Cornerstone would break the site again.
After going through the same steps several times we decided to update Cornerstone first through the dashboard before manually updating X using FTP and this seemed to do the trick.
I tried finding a thread that would help while I was troubleshooting and couldn’t find one that helped with my specific issue so I decided to drop a tip in here in case anyone else experiences something similar - figured we’d save you some grey hairs in the process!