I can't activate Safari 14 extensions even on Safe Mode: what to do now?

I am on an iMac, running the latest versions of macOS Catalina and Safari. I purchased Tampermonkey but it has been impossible so far to activate it: when I try to tick the box inside Safari's preferences I get the "Safari detected an app or service that interfered with clicking" dialog box. That directs me to an Apple help page which says booting on Safe Mode and activating it then will do the trick, but unfortunately for me, it didn't.

I have tried so far:

• Uninstalling Tampermonkey, booting on Safe Mode and installing it again; did not work.
• Reinstalling macOS; did not work.
• Wiping my entire HD and making a clean install of Catalina; did not work.

I have noticed this issue seems to affect most extensions, not only Tampermonkey, as so I have only been able to activate AdGuard — and that only happened because a bar popped down from the top of my Safari tab asking if I wanted to activate my newly installed extensions. If I try to manually tick the AdGuard boxes on Safari's preferences, it doesn't work, I get the same dialog box about interfering with clicking.

Does anybody have any light on this?