Stuck at bootloader

Looks like you maybe took the .621 update and then tried to SBF to a pre .621 build? If that's the case you're gonna need to flash a Milestone X SBF
Leftspin is correct the error your getting is because you were on 621 and if you want to root again you'll need to do the MX4.5.604 iso sbf that will also root you. It's all built into one file found here Linux solution to your Windows/RSD lite problems

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Ok so I did try this. However, it gets stuck on "waiting for phone" I waited 30 min and nothing changed.