How to keep my HID from permanently sending data

Hi there!
I've set up an HID which works fine with HIDClient.exe: it gets polled every 32ms (as per the descriptor), it receives and sends decent data. However, what I'm aiming at is something like:

1. Most of the time, the HID doesn't transfer any data
2. I set the HID to some kind of "Data Transfer mode". In this mode, it reacts to commands sent by the PC.
In a first attempt I'm trying to have the device transmit data at certain device-specific events. So I've set up a buffer which my device can fill. More specifically, I've changed usbuser.c:

Keil's example usbuser.c:

void USB_EndPoint1 (U32 event) {
  switch (event) {
    case USB_EVT_IN:
      GetInReport();
      USB_WriteEP(HID_EP_IN, &InReport, sizeof(InReport));
      break;
    case USB_EVT_OUT:
      ...
      break;
  }
}

My usbuser.c:

void USB_EndPoint1 (U32 event) {
  switch (event) {
    case USB_EVT_IN:
      while ((HID_BufferCtr>0) && (HID_BufferCtr<HID_BUFFER_SIZE)) {
        HID_BufferCtr--;
        InReport = HID_InBuffer[HID_BufferCtr];
        USB_WriteEP(HID_EP_IN, &InReport, sizeof(InReport));
      }
      break;
    case USB_EVT_OUT:
      ...
      break;
  }
}

What happens is that as soon as I increase HID_BufferCtr beyond 0, I get kicked out of the USB. Actually, HID_BufferCtr is even 1, so not even the amount of data changed from Keil's example. So what's the dramatic difference? All I'm doing is write some U8 to InReport, and afterwards it's the same call to USB_WriteEP.
I'd love to attach a USBlyzer log, but can't find the option. The log goes roughly like:

2* PnP Query Device Relations - Removal Relations (issued - failed)
2* PnP Query Device Relations - Ejection Relations (issued - failed)
4* PnP Query Device Relations - Removal Relations (issued - issued - failed - failed)
4* PnP Query Device Relations - Ejection Relations (issued - issued - failed - failed)
3* PnP Surprise Removal (issued - succeeded - issued)
2* URB Bulk or Interrupt Transfer (issued - canceled)
3* PnP Surprise Removal (issued - succeeded - succeeded)
3* PnP Remove Device (issued - succeeded - issued)
2* URB Abort Pipe (issued - failed)
3* PnP Remove Device (issued - succeeded - succeeded)

Now I can't imagine what in my changed code could cause the bus to disconnect me. Are there any ideas? Thanks in advance!

Best wishes,
Peter