I also encountered the same problem. This means that your server is sending "text/html" instead of the already supported types. After a little search, my solution was to add "text/html" to acceptableContentTypes set in AFURLResponseSerialization class. Just search for "acceptableContentTypes" and add @"text/html" to the set manually. Of course, the ideal solution will be to change the tpe from the server, but for that you will hade to talk with the server team.
I hope this helps you. Best regards and less bugs as possible in the code.
op.responseSerializer.acceptableContentTypes = [NSSet setWithObject:@"text/html”];
修改AFNetworking网络请求框架：
AFURLResponseSerialization.m文件
223行：
self.acceptableContentTypes = [NSSetsetWithObjects:@"application/json", @"text/html",@"text/json",@"text/javascript", nil];
加上蓝色部分，其实就是添加一种服务器返回的数据格式。
iOS"Request failed: unacceptable content-type: text/html"
原文：http://www.cnblogs.com/now-or-never/p/4772824.html