See my posts here and here, which may possibly help.
In summary, although the Roon 1.8 ↔ 2.0 Migration FAQ states clearly (in the context of a downgrade from 2.0 to 1.8 legacy):
You will need to install the Roon 1.8 legacy app on a Roon Remote/iOS device first before being able to take the downgrade.
what it does not say explicitly is that the only way Roon will know you have done that is by keeping open the version 1.8 legacy remote app when downgrading the Nucleus. I rather assume the same applies to anyone attempting to upgrade from 1.8 to 2.0. In other words, for those users, ensure you have the version 2.0 remote app open (i.e. attempting to connect to the Nucleus) when running the upgrade of the Nucleus.
Stephen