Rockbox

Try to refresh the windows drivers.
Under Device Manager, right-click the Rockbox device (maybe even do it for each sub device - Mass and HID, separately), and select ‘Uninstall’. Then unplug the DAP, and re-insert it (HID can be enabled).
I’ve noticed that Windows somehow caches the drivers, so when you upgraded your firmware, and now it also enumerates as a USB HID device, Windows can get confused.

Please report your results so we can know if this bug is valid or not.

Some more information:

HID disabled:
VID: 0×0781 (SanDisk Coorporation)
PID: 0×7421 Serial Number: 1000000000000000093B367880AE4FFF4

HID enabled:
VID: 0×0781 (SanDisk Coorporation)
PID: 0×7421 Serial Number: 3000000000000000093B367880AE4FFF4

Original Firmware, MSC:
VID: 0×0781 (SanDisk Coorporation)
PID: 0×7421 Serial Number: 00000000-00000000-8867b393-f4ffe40a-00000000

Original Firmware, MTP:
VID: 0×0781 (SanDisk Coorporation)
PID: 0×7420 Serial Number: 00000000-00000000-8867b393-f4ffe40a-00000000

So VIDs are equal for all, PID changes only with OF in MTP mode, and Serial Numbers are logically unique.
While I was on it, I checked, and when booting rockbox it always has the same PID (0×7421 in my case), even if the OF is set to MTP and has a different PID (0×7420 in my case). You probably already knew this as you wrote rockbox, but hey, it’s worth to make sure.

Okay, this is very weird. After going into the OF and setting it to MTP (and rebooting into Rockbox ofcourse), it works (HID and mass storage at the same time).
I went back into OF and set it to MSC, and it still works.
Tomer Shalev: I did that already, but I went from 3.4 to current, and both 3.4 and current have both HID and mass storage, so it probably isn’t some confusion because it now enumerate a HID as well (as it already did that with 3.4 stable, which worked for me flawlessly and continuously).

I am having the same issue on an e270r with r23746, but it started failing several days ago. I am not sure which current build was installed then. I first noticed the problem after connecting the device in the OF MTP mode, although this may be coincidental. I see the failure on Win 7RC and WinXP SP2. I have an e280v1 that is not having this problem.

I am attaching USBView info from the Win 7 system and pruned lsusb -v from my Ubuntu dev system of the OF MSC and MTP modes and the Rockbox HID enabled and disabled modes.

@tomers - I did uninstall the driver and then reattach on the Win 7 system and had the same results.

   USBView_e270r_OF_MSC.txt (1.6 KiB)
   USBView_e270r_OF_MTP.txt (1.8 KiB)
   USBView_e270r_RB_hid.txt (2.2 KiB)
   USBView_e270r_RB_nohid.txt (1.6 KiB)
   lsusb_e270r_OF_MSC (2.3 KiB)
   lsusb_e270r_OF_MTP (2.7 KiB)
   lsusb_e270r_hid (3.4 KiB)
   lsusb_e270r_nohid (2.3 KiB)