I've also seen this type of error when the owner of a manually-created folder is not correct. Sometimes (for whatever reason) the way an app is installed, be it Plesk, CPanel, or even the SSH / Bash user you are using to login to the server command line, will create a directory with a user that does not have the proper permissions.

Going into SSH and typing ls -l will show you who the owner is of each file and directory.

For instance I created a "drupal" user and a "drupal" group, that are allowed full permissions where needed. When in doubt I can chown drupal:drupal on any of these files or folders and that fixes permission errors that pop up (although I haven't had to do this in anything except some Perl scripts...)

Anyways looks like your /files folder is working again? I'd make sure the .htaccess file is still there.