I'm facing the same problem.
No way it can pair again with Mi Fit.
If I use Notify for Mi Band (I always use it without Mi Fit), it tries to complete flashing but it gets broken anytime. Then, after 3 attempts it just shows "Device is disconnected" error. So I try to restart my phone, and stgart again, but nothing actually changes.
So I tried to use Mi Fit again to start it all from zero, deleting and unistalling NfMB first, but Mi Fit cant pair my band, even though, device's bluetooth can see "Mi Band 5" around.
I went with original Mi Fit and Mod, but same thing happens. Now I installed NfMB again, and I could use Authorization code, so it could reach my band again, but firmware problem is still there.
Since problem started, the most noticeable thing is that my band could restart after flashing firmware (those times when procedure could pass step 1/2, but usually failed with step 2/2 or jumpingo directly to font), however, that screen shown by our friend ArielOrtega
was always there.
Tried the most of newer firmwares, beta included, with their respective resources.
What a strange coincidence that we flashed firmwares, the same day and just landed here to report the same error
Dear people, thanks in advance for your help.