I noticed that Avast Add-in for outlook works fine if I start Outlook with admin privileges.
But I use power user privileges usually and this prevents Avast Antispam Add-In for Outlook to work normally
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Wonderful - it SOLVED my problem! AND I did not repair or reinstall Avast. I did the following -
I started Outlook with admin rights, then closed and started with user (non admin) rights. Then I logged in Avast GUI into my account.
I do not know exactly, what helped indeed, but now Avast Antispam Add-In for Outlook is activated again