I had a unit with the same error. Upon inspecting the unit it appeared that the motherboard would not boot. I checked the capacitors and found two to be bad. This unit had barely any use so I suspect this is a weakness with the motherboard. I am sending the unit in to be repaired.
After sitting safely, warm and dry in its flight case for about five months, mine did the same thing. The unit is barely two years old, so there's NO EXCUSE for a capacitor problem! When I called Tascam tech support, and suggested the battery issue, the fellow intoned that it was "a repair issue", muttering something about capacitors under his breath.
The reason I tried to fire it up at all was because I'd just gone through the same battery deal with my client's RADAR. Got to keep the beasts plugged in to charge the backup battery (also true with my Studer A810!).
If this turns out to be a capacitor issue (unit is conveniently out of warranty) I'll let everyone know - and ROAST Tascam for not catching a bad batch of caps.
I have a Pentium 1/Win 95 tower that still boots fine - battery and motherboard ORIGINAL. There's just no excuse.