LaunchAgent in Objective C, Sandboxed App in Swift, how to connect

Hi,

the LaunchAgent is implemented with this (cutout):

@interface ServiceDelegate : NSObject 
@end

@implementation ServiceDelegate
- (BOOL)listener:(NSXPCListener *)listener shouldAcceptNewConnection:(NSXPCConnection *)newConnection {
    newConnection.exportedInterface = [NSXPCInterface interfaceWithProtocol:@protocol(pEpMacOSAdapterProtocol)];
    pEpMacOSAdapter *exportedObject = [pEpMacOSAdapter new];
    newConnection.exportedObject = exportedObject;
    [newConnection resume];
    return YES;
}

@end

int main(int argc, const char *argv[])
{
    ServiceDelegate *delegate = [ServiceDelegate new];
    NSXPCListener *listener = [[NSXPCListener alloc] initWithMachServiceName:@"foundation.pEp.adapter.macOS"];
    [listener resume];
    [[NSRunLoop currentRunLoop] run];
    return 0;
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
    <dict>        <key>Label</key>
        <string>foundation.pEp.adapter.macOS</string>
        <key>ProgramArguments</key>
        <array>
            <string>/Library/Application Support/pEp/foundation.pEp.adapter.macOS</string>
        </array>
        <key>MachServices</key>
        <dict>
             <key>foundation.pEp.adapter.macOS</key>
             <true/>
        </dict>
    </dict>
</plist>

The App is implemented with this (cutout):

func proxyErrorHandler(err: Error) -> Void {
    NSLog("%@", err.localizedDescription)
    statusText.title = NSLocalizedString("Connection failed", comment: "")
}

internal func applicationDidFinishLaunching(_ aNotification: Notification) {
    connection = NSXPCConnection.init(machServiceName: "foundation.pEp.adapter.macOS")
    if connection != nil {
        connection.remoteObjectInterface = NSXPCInterface.init(with: pEpMacOSAdapterProtocol.self)
        connection.resume()
        statusText.title = NSLocalizedString("Connecting…", comment: "")
        proxy = connection.remoteObjectProxyWithErrorHandler(proxyErrorHandler) as? pEpMacOSAdapterProtocol
        proxy?.subscribeForUpdate(downloadNotification: notifyDownload)
    }
    else {
        NSLog("pEpNotifications: %@", "cannot connect to pEp.foundation.adapter.macOS")
    }
}

I'm always getting proxyErrorHandler() called with "Couldn’t communicate with a helper application."

Any ideas?