I'm having the same issue... my mobile app works but not the full desktop.
Tried as a mobile version http://m.box.com  and worked, but is not the same...
I've already placed a support ticket.

Today, my Box folder disappeared so did Box from  KeyChain Access (I can no longer log in) .  I tried adding it, but I am not allowed.   I cannot log in.

This is on my old Mac with Sierra.

Can anybody help me?

Dani, Do you have a Free or Pay account?.
Try to log like this http://m.box.com maybe could help.

My account is free.  I can log in from my new computer with BigSur, but Box completely disappeared from the one I use at work (with Sierra), I cannot log in and there is no trace of Box in KeyChain Access.  I tried to add the log-in info in KeyChain, but I was not allowed to do it.

I succeeded in logging in from http://m.box.com    Thank you! 

I wonder what happened to the  Box on my desktop, though? Completely gone without my doing anything....

Is there way to restore it?

Dani