Avast reports on its product forums that Microsoft temporarily disabled the updates. Other sources indicate Microsoft made the decision due to complaints about blank desktop screens with no icons. At the moment, it's unclear what's causing this issue, as Microsoft and Avast are unable to reproduce the problem.
Microsoft temporarily disabled the updates on version 1803 on PCs with Avast. We are working together with MS to find out the cause of this issue, but unfortunately right now neither MS nor us are able to reproduce the issue.Avast users who wish to upgrade to the latest version of Windows will need to remove the software and wait for more news. Alternatively, you can delay the deployment of Windows 10 April 2018 Update