I don't know how, but yesterday the problem was suddenly fixed by itself. UI has been working fine since.
Windows7 x64 SP1 same issue. Machine took WIndows updates on Jan 9 UI failed to load. Downloaded off line installer of Avast Free Antivirus installed it same issue UI failed to load. Multiple reboots did not help.
Avast has a built-in program called AvastEmUpdate.exe which runs at every system boot and automatically fixes Avast related bugs or glitches. Which is why one sometimes needs to boot more than one time
while connected to the internet. A second reboot while connected usually sets the fix in place.
A standard practice I use is to do a clean install anytime the Avast version number goes up by one, e.g., from version 18 to 19, using bob3160's clean install method.
https://forum.avast.com/index.php?topic=224348.msg1489497#msg1489497 (Reply #11)
Too many changes are made in the course of updating version 18 during the time it is supported for bug removal and glitches. Version 19 is also an upgrade in protection abilities to protect against new malware, which is why it is being introduced vs. 18, but some old 18 files and registry entries can remain and affect proper operation of 19. These duplicate files from version 18 and then 19 are not meant to remain but it does happen. A clean install reduces the chances of a botched install of the new.
Some old files are not overwritten and replaced as they should be in an upgrade, as it were.
I always use the offline installer. I've found to be more reliable and it works better than the online installer. Plus, if I save the offline installer in a safe place, I can always safely revert to it in case the newer version fails to work out properly.
Online installers don't offer that option.