We've got Magento 188.8.131.52 on nginx/1.14.2 and PHP v7.3.14 and we're using EM Titany (free) theme. I've just moved the shop from an old server to a new one (PHP5 to PHP7) and things have been working on PHP5.
The theme comes with plenty of settings and a visual content editor. The settings are all working fine in the default admin area/style. The content editor only yields a 404 page without any exceptions being printed in nginx log and exception log.
I'm not sure where to start debugging. Usually issues like these might come from Nginx config but should then show something in the error log when it's a 404? (I can only see 200 entries in the access log when loading the 404 page). Or might it be from the theme which could be PHP7 incompatible (couldn't find any info on the EM website)?