Not sure if this was a problem but when trying to register to certain instances on my phone (Chrome iOS) the ball would spin forever. When doing it on my desktop it was no problem.
I think lemmy.ml is just receiving a huge amount of traffic all of a sudden. Even just a couple percent of all redditors suddenly making an account on Lemmy servers will give their comparatively smaller servers the beating of a lifetime.
I had a similar experience signing up to lemmy.world. I switched to a different password and was able to sign up. However, one of the commenters says that registration at lemmy.ml is closed.