Here’s the HTTP headers I see:
https://www.adonyawong.com/feed/
GET /feed/ HTTP/1.1
Host: www.adonyawong.com
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:48.0) Gecko/20100101 Firefox/48.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
X-Moz-Is-Feed: 1
Cookie: __ib2pgvar_260=a; wordfence_verifiedHuman=0892ff7d228d454e1f91a881eaea8769; wfvt_126474160=57b835e369eb2
Connection: keep-alive
Upgrade-Insecure-Requests: 1
HTTP/1.1 500 Internal Server Error
Date: Sat, 20 Aug 2016 10:50:20 GMT
Server: Apache
Etag: "1173c6dccebc9981a35907f015d3bab3"
X-Robots-Tag: noindex, follow
Link: <https://www.adonyawong.com/wp-json/>; rel="https://api.w.org/"
Expires: Wed, 11 Jan 1984 05:00:00 GMT
Cache-Control: no-cache, must-revalidate, max-age=0
Set-Cookie: wfvt_126474160=57b835ef127c3; expires=Sat, 20-Aug-2016 11:20:23 GMT; Max-Age=1800; path=/; httponly
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 1693
Connection: close
Content-Type: text/html; charset=utf-8
So, it’s actually responding with a 500 Internal Server Error. I think deactivating plugins one at a time is a good path to follow.