error :

Request failed: unacceptable content-type: text/html

or

The operation couldn’t be completed (Cocoa error 3840 )

fix this issue

 AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager new];
NSMutableSet *set = manager.responseSerializer.acceptableContentTypes.mutableCopy;
[set addObject:@"text/html"];
   manager.responseSerializer.acceptableContentTypes = set;