When trying to upload files to the Wordpress media library I encounter the following error: 

```
Unexpected response from the server. The file may have been uploaded successfully. Check in the Media Library or reload the page.
```

I enabled debugging mode in WP by adding

```
define('WP_DEBUG', true);
define('WP_DEBUG_DISPLAY', true);
define('WP_DEBUG_LOG', true);
```
to my wordpress config file.

After taking a look at debug.log there seems to be a problem with one of the WP core files `PHP Fatal error:  Uncaught Error: Cannot unset string offsets`

[The full debug log](https://gist.github.com/victorlane/e80ff2f97563b607687dd32036effacd)

The function throwing the error is `clean_dirsize_cache`

Line 8381: `unset( $directory_cache[ $path ] );` 

I've already tried the following:

 - Allocated more memory in php.ini
 - Changed limit for max file sizes

The error happens on every file I try to upload, file size or file type doesn't seem to matter. 

As I'm not experienced with Wordpress and I haven't worked on this site previously at all I'm not quite sure what caused this to start occuring (I have no prior knowledge of the site nor