Try this. Edit your /etc/php.ini and look for a line:
file_uploads = On
Mine was set to Off and that's when I got the messages "cannot access these files directly" from add_photos, and "protocol out of date" from gallery_remote.
This switch allows you to do http file transfers. I didn't have this problem with Red Hat 7.1, but when I built a new server on RH 7.3, it suddenly stopped working - so it's possible by default that the RH7.1 has it set to "On", and RH 7.3 defaults it to "Off" on the initial setup