It's the same error as in another thread (problems with auto-updates v731), so try this too:
Edit setup.ini file in avast4/setup folder, add the following lines:
[BufferLog]
Enable=1
Limit=0
Run update again (it will probably fail), and send me files Setup.ini, SetupDbg.log, Setup.log and your license key (only the letters before dash separator) to my email (forejt@avast.com)