OK so the first thing to do is to uninstall completely. Your issue might come from having upgraded directly from 4.8 to 5.0. So first uninstall, reboot, and reinstall V5 from scratch. I can't recommend to use the new uninstall utility for V5 , it's in early stage of development so just use the normal command in Windows for this. Also, see if there's anything left in Alwill folders in program files, and in all users or program data, just delete manually what's left after a reboot, and clean your temporary folders (use CCleaner or else). Once done, you can reinstall V5.
Or I phrase again my question : how short before the expiration date that avast! allow us to re-register our registered avast!
you'll be notified by mail I think, or may be a pop up from the application... but just try to follow the instructions I gave you