Migration from Lightroom Classic to Lightroom - ERROR? WHAT?

Hi,

I am trying to migrate my Lightroom Classic CC catalog to the new could-based Lightroom.

From the new Lightroom's File menu I choose "Migrate Lightroom Classic Catalog...", and I immediately get a message box that seems to (automatically) find the catalog I want to import, correctly identifies its size, but gives the error:

"Cannot stop or continue the pending migration. Please login to the user account from which the migration was initiated"

This is baffling because

a) This is the first time I've started a migration (so there should not be a 'pending' migration).

b) I only have one adobe user account, which I have used for the duration of my time with Lightroom Classic, and now with the new Lightroom.

Can anyone in the community or in Adobe support please help with this issue so I can migrate from LR Classic to LR?

I've read the related migration tips and FAQ pages, none seem to address this issue.

Thanks!