As announced for my phone, on which Opera only worked if Secure Folder was disabled, I proceeded to test the solution proposed by miyukiwork/efolch:
- reactivated Secure Folder;
- tested Opera, which started crashing again;
- deactivated the "Mobile Services Manager" system app;
- re-tested Opera which worked flawlessly.
I therefore confirm the effectiveness of the solution.
Thanks to all for the contributions.