Ok, I'm not sure what happened, but I think I may have killed one of my S2s last night. I did a full backup of the existing install, then dirty flashed over it. On reboot, all I got was an unprecedented spinning broken circle and a bootloop. It would flicker, then spin the circle some more until I power cycled the phone.
I went back into recovery, did partial wipe, and this time I actually saw the usual spinning dots, but the device never booted. I let it sit spinning colored dots for about 10 minutes. Weirdly, when I attempted to power the phone down, it started flashing the LED (camera flash) several times before rebooting (instead of shutting down) So, back into recovery, where I restored the backup. I rebooted, but now I'm back to the spinning broken circle.
I then did a full wipe of everything but the SD card and a restore, but I'm stuck with the spinning broken circle. All I can boot at the moment is a recovery.
Any ideas? Also, given all the wiping, is there any chance of a log available in TWRP? Lastly, I'm using TWRP 2.8.6.0 if it matters.
UPDATE:
Ok, I got it back. I nuked everything on the phone except /sdcard and reinstalled. I then did a full factory reset prior to first boot. It took a while, but I'm now in account setup.
Glad you finally got it working. Kind of scary, though, because it means your nandroid either failed to create correctly or failed to restore correctly.