I managed to get through to F Secure on chat - this will be addressed in the new update of the software due to release shortly.
Hi all, I tried with F Secure online support (very kind and supportive) and finally I discovered that the problem can be the MagicPrefs app (if you use it, to tweak the Apple Magic Mouse).
For installing both the browser extension and to fix the new security block in Mac OS X High Sierra, you have to temporarily disable it. This will make you to click the "allow" button in Preferences-Security.
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