First, there are already other topics with this, so please next time search and read them before re-posting. If it would be necessary, then post accordingly (in the same topic or a new one, depending on several factors).
Second, many things can cause this type of messages from Avast. For whatever reason, Avast is not able to read the contents of those files. If you have some file with password protection, Avast has no way to know which password you are using, so there is no way for Avast to open it if it needs to.
Something equivalent (but a little bit different) would happen if you have compressed some file with some proprietary compression algorithm and no publicly known way to decompress it. Avast would have no way to look into this archive so to scan it. If Avast were to know, somehow, that this would be an archive, it would tell you that the compression method is unknown or unsupported.
It is not saying that there is some malware. It is saying that it doesn't have the means to "go beyond". That's the point of password protection, isn't it?
In any case, IMO Avast should investigate these several reports about this new behaviour with Secunia, so to be sure it is not a bug or a problem with some update in Avast.