I have made a copy of my live site M2.4.2 to a dev site subdomain on the same server.
I copied all the files,
created a new database & imported a dump from the original
Changed core_config_data to the new subdomain
Changed app/etc/env.php to new database details & also downloadable domains to new dev site.
Deleted var cache, page cache & logs
run: php bin/magento cache:flush & clean
Website appears to be ok but when I log into admin only Dashboard & Find Partners & Extensions links work on the left menu. Also if I click onto the username used to log in in order to log out instead of giving me a sub menu it goes to the account information page.
Someone suggested deleting the var folder all together & running: