I had this problem as well, also impacted image uploading in messages.

First updated to the newest nodebb release (1.11.0) and Node.js stable @ (11.2).

Tried to downgrade Node.js back to (10.12) with no love.

Returned to Node.js stable (11.2), and fixed by simply building sharp locally.

npm install --build-from-source sharp

Cheers
Anjin

I was able to get sharp working by simply building it locally.

npm install --build-from-source sharp

The only thing missing on my minimal centos 7 image, was g++.