Same problem here...
An security update brought me this issue. I tried to flash it with OST but it was unable to rewrite the bootloader (also in emergency mode). Then I used "fast boot" to unlock the bootloader. ERROR / Not allowed.
I also tried to update the firmware with "fast boot" but it didn't work as the bootloader is locked. I also changed the active partion by "fast boot". So I was able to reach the factory reset mode in my phone. I followed the menu and it did some reboots => still stuck in the "download mode". Any other ideas? Thx a lot