Disallowed Key Characters

When I try to request **https://api.coinigy.com/api/v1/markets** it recently started to return this:
 **Disallowed Key Characters.**

The returned header is:
**Server: nginx
Date: Thu, 05 Jul 2018 17:41:05 GMT
Content-Type: text/html; charset=UTF-8
Transfer-Encoding: chunked
Vary: Accept-Encoding
Content-Encoding: gzip**

What does it mean?