Hello!
I have faced with issue second time and looks like it can be repeated - after fresh installation of last Cura on blank system (new PC), there is no option for restore backup at start - you need select at least one printer (even after sigh in dialog), only then, its possible to go backup restore plugin and select backup.
But then I get second problem - Cura closing, but when I start Cura again (normal restore procedure), there is again "Add a printer" dialog, and there is no way to ignore that. After selecting some new printer, if I go to machines list - there will be my machines from backup + added one. If I delete last, Cura working normally with my settings, and no "Add a printer" window at next time. But if I try to restore same backup again - Add a printer dialog appear back..
For example here you can see that I was signed-in, but Add a printer appears
even if I have printers from backup: