It looks like SmartThings used the migration to Samsung SmartThings to enforce stricter region rules depending on the actual physical location of the Hub (based on its IP address).
I am in France using and old v1 Hub, initially registered into the US cloud, but now nothing works anymore : neither the SmartThings Classic App, nor the new Samsung one, within which I cannot register my US v1 Hub.
After 2 weeks of useless iterations with SmartThings support, the only conclusion was that I could as well scrap my $2000 worth of Thingsā¦
Great benefit of a cloud based solution : you cannot even migrate from one region to another !
Surely a side-effect of the COVID-19 virus containment measuresā¦