serialization of Base64 string in JSON payload with HessianKit (Objective-C/Cocoa) - objective-c

Question

I'm trying to connect my iOS-App to an existing Grails backend server. The backend exposes a hessian webservice by using the grails remoting plugin (version 1.3). My Android app successfully calls all webservice methods.
My goal is to transmit a jpeg image from the phone to the server (works with the Android app). My approach is to use create a JSON object with JSONKit and include the image as a base64 encoded string. I'm using HessianKit in an XCode 4 project with ARC targeting iOS 4.2 and Nick Lockwood's NSData+Base64 categories for Base64 encoding (https://github.com/nicklockwood/Base64).
Here's my code:
NSMutableDictionary *jsonPayload = [NSMutableDictionary dictionary];
[jsonPayload setObject:[theImage base64EncodedString] forKey:#"photo"];
NSString* jsonString = [jsonPayload JSONString];
NSURL* url = server_URL;
id<BasicAPI> proxy = (id<BasicAPI>)[CWHessianConnection proxyWithURL:url protocol:#protocol(BasicAPI)];
[proxy addImage:jsonString];
The problem is that the server throws an expection when called by the app:
threw exception [Hessian skeleton invocation failed; nested exception is com.caucho.hessian.io.HessianProtocolException: addImage__1: expected string at 0x7b ({)] with root cause
Message: addImage__1: expected string at 0x7b ({)
Line | Method
->> 1695 | error in com.caucho.hessian.io.HessianInput
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| 1681 | expect in ''
| 1473 | parseChar in ''
| 792 | readString in ''
| 181 | readObject in com.caucho.hessian.io.BasicDeserializer
| 1078 | readObject in com.caucho.hessian.io.HessianInput
| 300 | invoke . . in com.caucho.hessian.server.HessianSkeleton
| 221 | invoke in ''
| 886 | runTask . in java.util.concurrent.ThreadPoolExecutor$Worker
| 908 | run in ''
^ 680 | run . . . in java.lang.Thread
All other JSON payloads from my app (Strings, dates, numbers, etc.) can be deserialized by the server without any problem and the other way round, i.e. sending a base64 encoded image as json payload to the app from the server as a response also works.
After spending hours reading bug reports and mailing lists, I suspect that the problem might be that HessianKit only supports the Hessian 1 protocol but the hessian version shipped with remoting 1.3 is 4.0.7. 4.0.7 probably uses the Hessian 2 protocol and isn't compatible backwards. But that's just guessing.
EDIT: Actually, the issue has nothing to do with JSON. The same exception is thrown when I just pass the string as a normal string (and not embedded in JSON) to the webservice.
Has someone experienced a similar problem and knows a solution?