PMaff, you can test this or that...
Right now, after all this mess, I suggest an installation from scratch:
1. Download the latest version of avast! Uninstall Utility and save it.
2. Download the latest avast! version and save it.
3. Uninstall avast from Control Panel (if possible). If, for any reason, you can't run it, try booting in Safe Mode and doing it from there. Anyway, boot after that.
4. Run the avast! Uninstall Utility saved on 1. If, for any reason, you can't run it, try booting in Safe Mode and doing it from there. Anyway, boot after you've run it.
5. Install avast! using the setup saved on 2. Boot.
6. Register your free copy or add the license key for Pro. Or even upgrade your key from old versions.
7. Check and post the results. If, for any reason, you did not solve, try doing the step 3 in Safe Mode anyway.
I tried:
1. booted to safe mode at first, uninstalled Avast without any problems
2. reboot
3. downloaded that avast! Uninstall Utility on another machine,
copied it to the UBS stick
then put it on the Windows 7 machine.
4. reboot to safe mode as said on the webpage of the Uninstall Utility.
Uninstall Utility showed the correct directory and uninstalled without
problems.
5. reboot
6. installed the already downloaded 6.0.1367 with md5sum
b5d0d7c09c5d34062768386cc62fb47e
which is the same when you download it now.
7. when installing I use the key that I always used
During install I get one message (of course, I am offline but why
does Avast need some server at this point?):

but installation goes on and says "Successful" at the end.
8. reboot
9. install of updates from 2011DEC16
10. reboot
11. avast shows that update is from 2011DEC16
12. tried to install update from 2011DEC18, get message that all is up to
date (behaviour is still the same) so the error is not solved
13. reboot
14. try the update from 2011DEC30 and I got no ready message.
Avast still says the last update was 2011DEC16
So the error is not resolved by this erase/install cycle.

Edit: wait a minute.
When looking at the logs I see that after installation Avast says:
30.12.2011 16:17:13.000 1325258233 (null) Started: 30.12.2011, 16:17:13
30.12.2011 16:17:13.000 1325258233 (null) Running setup_ais-557 (1367)
30.12.2011 16:17:13.000 1325258233 (null) Operating system: Windows 7 ver 6.1, build 7601, sp 1.0 [Service Pack 1] x64
and when I start the 2011DEC16 update it says:
30.12.2011 16:21:47.000 1325258507 (null) Started: 30.12.2011, 16:21:47
30.12.2011 16:21:47.000 1325258507 (null) Running setup_ais-557 (1367)
30.12.2011 16:21:47.000 1325258507 (null) Operating system: Windows Vista ver 6.0, build 6000, sp 1.0 x64
Is it possible that the 2011DEC16 update runs ok but ruins it all?
Edit: I've redone the above points.
Difference: in the above I installed the English version so that you don't have
problems reading the dialogs. The English Version wants some server when installing.
This time I tried the German version (as before) and there was no question about some server, which is strange.
This time I omitted update 2011DEC16 and tried 2011DEC30 at first.
2011DEC30 did not display any success message and it did not
put something into the logfile.
Tried 2011DEC18 afterwards and got the bug
http://www.imagebanana.com/view/x9c1mf9m/ErrorcodesUpdate18122011.jpg2011DEC18 wrote into the logfile that the OS is Windows Vista.