I can verify that the solution posted by Stone3t has worked for me (disconnecting the CMOS battery). Too bad it requires disassembly of the back case which may be too advanced for average users.
I had all the symptoms / problems described by posts all over Dell's sites as well as others: can't update BIOS, unit won't sleep, etc. Corrupt ME was the key. (Intel Management Engine firmware) Stupid thing is: it was corrupted through Dell's own updates.
Calling the repair depot, or any other Dell support (and being transferred 5+ times, waiting on hold for hours) was utterly useless as they refused to help because my unit was "out of warranty" even though some had mentioned there was a recall on it.
I'm now updated to A15 and through all windows updates and everything appears to be working fine again. No thanks to Dell support who refuse to share the answer.
I will not be buying any Dell mobile devices in the near future.