I just had this issue.
My OS is Ubuntu 10.10
I downloaded the .deb today, installed, entered my key.
At this point the gui worked fine.
Virus database was old, so updated.
At the end of the update, 2 errors.
Now the gui and cli don't work.
avast! log file; version 1
2 2011-12-27 15:02:32 An error occured in avast! engine: Invalid argument
0 2011-12-27 15:02:32 Updated virus database to version 111227-2, 12/27/2011.
2 2011-12-27 15:03:05 An error occured in avast! engine: Invalid argument
3 2011-12-27 15:03:05 Error while initializing avast! engine: Invalid argument.