Hello @wbc,
Thanks for asking. 
Please try out following general solutions.
- Please make sure that you are running PHP 7 or later on the server as running old version of PHP can have potential issues. You can refer following document for server requirement https://wordpress.org/about/requirements/1
- Remove custom CSS, Javascript and templates.
- Reset your htaccess file by renaming it to .htaccess-bak. Then in WP Admin Menu, go to Settings > Permalinks and just click the Save Changes button.
- Contact your host to increase your allocated memory or do it yourself by adding this code in your
wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
If nothing helps, please give us WP Admin and FTP access in a Secure Note.
Thanks.