Hello everyone, just for future reference for others who may find in the situation described above.
Everything was solved by simply going back to the Administration tab, finding my theme not present. So I went to the Plugin Gallery and I noticed that the theme as well had to be upgraded (in my case Immersion). The upgrade procedure of the theme was just a matter of a click and went smoothly.
Now I have the Journal updated to the very latest OJS release, and it feels really very good.
I dedicate this Upgrade to Luna Rossa, which has navigated with great elegance during the last days.
2 Likes