Run the BCU seperatly and not through the smart scan.
You should see what is found that way.
If nothings shows up when you click to show the log/information, it is either a bug or a problem with your avast installation.
Perform a repair of avast and see if the problem is solved.
If not, perform a clean installation of avast.
For the log file, you can always navigate to it and open it manually.