Yeah, the 500 request is fine and is to be expected. I also went to my Firefox (54) as well and tried it out and it does work for me.
If you click on the triangle icon next to the error response, you should be able to see a bunch of different sections. One of them is Response.
https://drive.google.com/open?id=0B39jDgCtzVHuQ0tZOHVWQXhhUGs
The above link shows the typical Javascript code that runs on the 500 response, to include hiding the vote links and unhiding the unvote links.
The following are from the Headers section (far left) for additional comparison, with some information being redacted.
Response Headers
Cache-Control: no-cache
Connection: keep-alive
Content-Type: text/javascript; charset=utf-8
Date: Mon, 31 Jul 2017 00:33:22 GMT
Server: nginx/1.11.3
Transfer-Encoding: chunked
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-Request-Id: ddc9fa8f-1f9d-4cc8-b78d-59ea5a548e08
X-Runtime: 0.010692
X-XSS-Protection: 1; mode=block
Request Headers
Accept: */*;q=0.5, text/javascript, application/javascript, application/ecmascript, application/x-ecmascript
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.5
Connection: keep-alive
Content-Length: 0
Cookie: *REDACTED*
DNT: 1
Host: danbooru.donmai.us
Referer: http://danbooru.donmai.us/posts/2803769?tags=girls_und_panzer
User-AgentMozilla/5.0 (Windows NT 6.3; WOW64; rv:54.0) Gecko/20100101 Firefox/54.0
X-CSRF-Token: *REDACTED*
X-Requested-With: XMLHttpRequest
Checking the Params section (next to Headers), it should be either "score up" or "score down".
If you don't see anything off comparing yours to mine, try using a different browser, or resetting Firefox.
