Hi there, further to the above I’ve heard back from the Dev team who’ve advised the following in case it can help further.

For the htaccess file; this is what it should be like:

# BEGIN WordPress

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

# END WordPress

If the install is in a sub-directory then this changes, replace the above lines with the modified ones below:

RewriteBase /
RewriteBase /your_wordpress_directory/
and also change

If that doesn’t fix it then the best thing to do would be a reset. If the core files were edited then there’s no way for us to tell what went wrong. Make a backup (ftp and db) and reinstall WordPress with a new database. Then you can always import the same content and even the db if that wasn’t the issue. Clean start is what we’d suggest if WordPress and the DB isn’t functioning as expected.