I had the same problem today.
I tried multiple times to
- re-flash TWRP
- flash older versions of TWRP
- flash older versions of the factory image (without the internal storage part)
- flash the latest factory image (without the internal storage part)
Nothing changed, but this procedure worked for me (only once):
- Start the phone in
- On the PC, rename latest TWRP in
recovery.img (I think it isn't required, but that's what I did)
On the PC
fastboot boot recovery.img
I backed up everything I needed from the internal storage (via
adb) and then rebooted the phone.
I was expecting to do a full wipe, but everything started as usual.
EDIT: Sadly this procedure works every now and then. I noticed that leaving the phone boot loop, for example during the night, brings it back to a bootable state, but if you reboot the device the boot loop sequence starts again.