Cannot upgrade WordPress Plugins on Synology DiskStation?

I have WordPress 4.1.1 running on my Synology DiskStation 1512+ with DMS DSM 5.1-5022 Update 3 and it’s working great! However, I’m having a problem updating one of my WordPress plugins from within the WordPress Dashboard.

I’ve been in touch with the plugin developer and he seems to think it’s some sort of configuration or permissions issue: The error message I’m receiving is below. Is there something I need to change on the DS?

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Downloading update from https://downloads.wordpress.org/plugin/ … .2.9.3.zip…

Unpacking the update…

Installing the latest version…

Removing the old version of the plugin…

Could not create directory. /web/wp/wp-content/plugins/ninja-forms/

Plugin update failed.
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