I think something is wrong with my proxy setting... that seems to be the reason why Avira will not update. I have also uninstalled Astrill, but it didn't help. Here's a log from Avira:
Avira Free Antivirus Updater
Complete product update
Creation time: Wednesday, November 21, 2012 10:58:48
Operating system:
Windows 7 x64 () [6.1.7600] 64 bit
Product information:
Product version: 12.0.0.898
Updater: C:\Program Files (x86)\Avira\AntiVir Desktop\update.exe 12.1.13.17
Update resource: C:\Program Files (x86)\Avira\AntiVir Desktop\updaterc.dll 12.1.0.17
Library: C:\Program Files (x86)\Avira\AntiVir Desktop\update.dll 1.0.0.8
Plugin: C:\Program Files (x86)\Avira\AntiVir Desktop\updext.dll 12.1.0.17
GUI: C:\Program Files (x86)\Avira\AntiVir Desktop\updgui.dll 12.1.3.17
Temp Directory: C:\ProgramData\Avira\AntiVir Desktop\TEMP\UPDATE\
Backup folder: C:\ProgramData\Avira\AntiVir Desktop\BACKUP\
Installation Directory: C:\Program Files (x86)\Avira\AntiVir Desktop\
Updater folder: C:\Program Files (x86)\Avira\AntiVir Desktop\
AppData folder: C:\ProgramData\Avira\AntiVir Desktop\
Proxy settings:
System settings used
10:58:48 [UPD] [ERROR] Terminating update. Initialization of UpdateLib has reported error 10107.
Summary:
********
0 Files downloaded
0 Files installed
Wednesday, November 21, 2012 10:58:48
The update failed!