Upgrade to 12c: ORA-38753: Cannot flashback if there is tablespace with FLASHBACK=NO !

Yesterday at customers site I had to upgrade one 11g database to 12c and stumbled upon a tricky issue. As I already upgraded a many dozens of databases, I think (or I thought at least), that I am pretty familiar with the whole process: planning, preparation, check list, pre-upgrade tasks and so on… But, yesterday ...