My client's copywriter contacted me as they were unable to upload images for their posts in their WordPress website. After trying a few suggestions to resolve this, I still couldn't find the cause of the error message. Here's how it played out:

Run out of web server space?

My immediate thought was that they had run out of disk space on the file server. A quick check in the hosting account confirmed they had heaps of space, so on we go.

A plugin issue?

A fairly standard approach to solving mysterious issues in WordPress is to disable all plugins and see if the issue persists. Unfortunately, on this occasion, the problem continued with all plugins disabled.

When you disable all plugins, some/much of the website's functionality is also disabled so it is something you need to do quickly and probably not at the peak activity time for the website. In business websites, this is best done after hours and certainly not at typical peak times 10am Monday to Friday.

Steven Stern suggests installing the Health Check plugin then using its troubleshooting capability to disable plugins and themes without impacting visitors.  Nice work, Steve. Refer: WordPress Support Forum

Other responses from this thread suggested updating PHP, setting file permissions etc

When all else fails...

As I'd reached the end of my technical abilities, and need to get this fixed quickly I contacted Siteground on their support chat, and within seconds their support specialist identified that php.ini had been recently edited to block file uploads:

set file_uploads=no

 After editing php.ini the file uploads started working again.

Its fixed, but how did it happen?

Its one thing to identify and rectify the cause, but a completely other activity is to find out why it happened. The last thing I need is an unstable website that has a mysteriously re-occuring issue.  

As it turns out a security plugin had been installed by another agency not appreciating the impact on other functionality within the website. All good intentions there, but when the security lockdown is so tight that the client isn't allowed image uploads, then its way over the top!

Mental note: Reduce that agencies access level so they can't break the site going forward!