Hard-coded path somewhere in my WordPress install?

Because I’m moving my site to a new hosting provider, I’m testing the transfer of my WordPress install. Both providers use the same DirectAdmin management tool, and I’ve downloaded a full backup from the old provider and restored it to the new provider, as recommended by the provider.

Testing the transfer, I’ve already found that my db name and user have changed, and corrected that in wp-config; and that WordFence has added an absolute path to my .htaccess, and I’ve removed that (I disabled WordFence prior to creating the backup, but that turns out not to have been enough.)

With both these corrections, the moved site still gives errors, countless of them in fact: when I modify my local hosts file to send me to the new IP, and browse to the domain, I get an ever-growing page of these errors:

Warning: is_dir(): open_basedir restriction in effect. File(/home/<oldusername>/domains/harlandawards.eu/public_html/wp-content/uploads/2016) is not within the allowed path(s): (/home/<newusername>/:/tmp:/var/tmp:/usr/local/lib/php/:/usr/local/php56/lib/php/) in /home/<newusername>/domains/harlandawards.eu/public_html/wp-includes/functions.php on line 1616

The line 1616 referred to is in the function wp_mkdir_p, which accepts any path $target as argument and attempts to create the target path if it doesn’t exist. Apparently, this function is called from somewhere with a path from the old installation.

Is there a way to fix this, or is the backup-restore method not actually a valid way to move my site?