Sending params with POST Req in JSON with AFNetworking - objective-c

Question

I have a small problem with AFNetworking.
I'm not able to send the parameters and the data to my server (php-Skript)
The data (from NSDictionary) have to be JSON.
Ignore the senseless code parts please.
I'm receiving Errors like:
NSLocalizedDescription=Request failed: unacceptable content-type: text/html}
or Error: Error Domain=com.alamofire.error.serialization.response Code=-1016 "Request failed: unacceptable content-type: text/html"
NSDictionary *parameter = [[NSDictionary alloc]init];
parameter = #{#"device-id": #"iOSDeveloper1234567432",#"system": #1,#"token": #"iOS-TestToken",#"mail_enabled": #"false",#"mail": #"NULL", #"movies": #[#"matrix", #"matrix2", #"matrix3"]};
NSError *error;
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:parameter options:0 error:nil];
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
NSURL *url = [NSURL URLWithString:URL];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:60.0];
[request addValue:#"application/json" forHTTPHeaderField:#"Content-Type"];
[request addValue:#"application/json" forHTTPHeaderField:#"Accept"];
[request setHTTPMethod:#"POST"];
NSString *stringkey;
stringkey = (#"k=");
stringkey = [stringkey stringByAppendingString:APIKEY];
NSString *strings;
NSString *myString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
myString = [myString stringByAppendingString:(#"&k=")];
myString = [myString stringByAppendingString:APIKEY];
NSLog(#"%#",myString);
myString = [myString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
request.HTTPBody = [myString dataUsingEncoding:NSSymbolStringEncoding];
NSData *postData = [NSJSONSerialization dataWithJSONObject:parameter options:0 error:&error];
NSLog(#"%#",error.localizedDescription);
AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
[manager GET:URL parameters:myString progress:nil success:^(NSURLSessionTask *task, id responseObject) {
NSLog(#"JSON: %#", responseObject);
} failure:^(NSURLSessionTask *operation, NSError *error) {
NSLog(#"Error: %#", error);
}];