Just noticed that it's sent on a 404 on vagrant:
vagrant@mediawiki-vagrant:~$ curl -I http://127.0.0.1:8888/wiki/dev/thumb/9/94/Jabrayil_in_Azerbaijan_%28semi-secession%29.svg/120px-3so5znk9zije2xf2eug20xndxblelnn.png
HTTP/1.1 404 Not Found
Server: nginx/1.4.6 (Ubuntu)
Date: Fri, 18 Nov 2016 17:12:44 GMT
Content-Type: text/html; charset=UTF-8
Content-Length: 0
Connection: keep-alive
Content-Disposition: inline;filename*=UTF-8''Jabrayil_in_Azerbaijan_(semi-secession).svg
Wikimedia-Path: thumbor/9/94/Jabrayil_in_Azerbaijan_(semi-secession).svg/120px-3so5znk9zije2xf2eug20xndxblelnn.png
Wikimedia-Thumbnail-Container: wiki-dev-local-thumb
Thumbor-Parameters: {"width": "120", "image": "http://127.0.0.1:8040/v1/AUTH_testproj/wiki-dev-local-public/9/94/Jabrayil_in_Azerbaijan_(semi-secession).svg", "filters": "format(png)"}
Xkey: File:Jabrayil_in_Azerbaijan_(semi-secession).svgAlso, for SVG it should have .png added, it's a mistake to make it the original's filename.