As far as I’m aware there is no Avast interaction on this (or you would have had an alert or message).
Where are you searching from ?
A screenshot and any error message you are getting may help.
A windows update may have been involved, you say your copy of win10 is fully updated, if so apparently searchUI.exe has been changed/renamed to SearchApp.exe, but this happened some time ago. I did a search on searchUI.exe and got this attached image at the top of the listings.
EDIT: if you are searching from the Magnifying Glass icon on the system tray, that works on my system.