You can have avast ignore them in the future (just that the option you tried it a one off action), but you should first confirm it actually is an FP and then report it to have the FP corrected for 'all' avast users, that is the best way to go about it, see #### below.
avast can do nothing about password protected files and this isn't unusual.
See
http://forum.avast.com/index.php?topic=35347.msg297170#msg297170 this topic for more information on why files can't be scanned.
Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.####
You could also check the offending/suspect file at:
VirusTotal - Multi engine on-line virus scanner and
report the findings here the URL in the Address bar of the VT results page. You can't do this with the file securely in the chest, you need to extract it to a temporary (not original) location first, see below.
Create a folder called Suspect in the C:\ drive, e.g. C:\Suspect. Now exclude that folder in the
Standard Shield, Customize, Advanced, Add, type (or copy and paste) C:\Suspect\* That will stop the standard shield scanning any file you put in that folder. You should now be able to export any file in the chest to this folder and upload it to VirusTotal without avast alerting.
If it is indeed a false positive, see
http://forum.avast.com/index.php?topic=34950.msg293451#msg293451, how to report it to avast! and what to do to exclude them until the problem is corrected.