Shimo
I ran into this with a client's installation and was able to fix it quite easily. I did a few things, and I'm not sure if all of them were necessary, but here is what I recommend doing:
Change the Base folder setting from your current relative path (file_attachments) to the absolute path on the server, which will look more like this:
/var/www/vhosts/your-domain.com/support/file_attachments
If that doesn't fix the issue, grab the latest copy of the plugin, upload it to /includes/plugins, go to Admin Panel > Manage > Plugins and disable and then reenable it, and you should be good to go.
That's what worked for me, but KevinTheJedi has a good point about permissions.
Make sure that the file_attachments directory has is owned by the same account:group that your main website runs under.
If I go to my main web root directory and run ls -l I can see what account/group owns the files
drwxr-xr-x. 9 rede psacln 4096 Jun 3 2018 wp-admin
drwxr-xr-x. 11 rede psacln 4096 Jul 20 13:35 wp-content
drwxr-xr-x. 18 rede psacln 12288 Jun 3 2018 wp-includes
In my case, the files are owned by account rede and group psacln.
Therefore, I can set the correct ownership like so:
chown -R rede:psacln support/file_attachments
Mind you, if you use Ubuntu you'll have to set both the owner and group to www-data, like so:
chown -R www-data:www-data support/file_attachments
