Thank you for your answer, jm4444.
No direct solution, great. 😕
Got a look at it and I read, "Please note that the Export / Import process ignores nested folders, so any existing nested folders will simply be flattened to regular folders." It means that I'll have to reorganise ALL my subfolders? Everything will be mixed up and scrambled? Greater. 😞
I really have so much free time and I don't have any ideas on the ways to fill it. Thanks Opera devs, you made my day. AND my night.