Even though we have CloudFlare DDOS protection, it looks like our site is being targeted, probably by bots. I can't SSH into the site anymore, so it appears some crackers have found a security bug which they have exploited, and that they have logged in.
Just as a preliminary measure, I reccomend that you download all images which you have uploaded to imgtc if you do not have a copy. The hackers may be SJWs, and in that case they would probably delete the images.
I have contacted our hosting service's security team, who should hopefully return me access and allow me to apply some patches.