That seems strange that if you disable Jetico it works or with Jetico enabled and do a manual update that works but not automatic updates. I would expect it not to work for either auto and manual updates as the process is the same, just the initiation is different.
What is in the logs, avast! Log Viewer, anything more detailed ?
you should only need access for avast.setup, ashMaiSv.exe (avast! mail scanner) and ashWebSv.exe (avast! web shield).
You could try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this. Not sure if this would make a difference since the manual update works.
You could also try deleting the entry for avast.setup in Jetico pf and try updating again, this would force Jetico to ask again and possibly correct a permissions data error.