I just encountered the same issue, not a theme problem as I originally thought.
I hope this fix applies to you or anyone else that has experienced it.
The issue seems to have appeared once the URL in the database is changed, while in the process of migrating over to a new hosting space.
The old site works fine because the old URL has caused wordpress to expect a certain number of characters from the URL.
eg. Old URL: https://johnsmith.com = 20
New URL: https://johnpaulsmith.com = 24
(Export a copy of your database for safe keeping)
Change the URL within wordpress (wp-admin > Settings > General) to force WP to update the expected string within your database.
eg. https://dohnpaulsmith.com, still 24
Change back to the correct URL within phpMyAdmin.
This worked for me, I hope it works for you.