Hello again!!!
So, because I wasn't satisfied with the randomness of my problem's solution, I checked a little more the whole situation.
I uninstalled SVP 3.1.6 once more, I chose to delete the SVP settings folder, I manually checked for any files remaining on the hard disk, I run CCleaner to remove any registry leftovers, I restarted my computer and reinstalled SVP 3.1.6, choosing to install the settings folder for all users.
And, as before I cannot change the Interface from the system tray. In fact, I cannot change any settings from the system tray.
I located the SVP settings folder and the file SVPMgr and manually changed the Interface selection value to "2"
That forced the Expert Interface and I have access to the advanced settings, but the fact remains that I cannot change any other setting from the system tray Interface. Processing threads, frame crop, outer lighting etc, all remain unchanged to default values.
But I can change the active profile settings (by double clicking on the SVP system tray icon) when a video file is playing.