As I already said in another thread today, the original message is complete nonsence.
AV Software doesn’t have any troubles in seeing the file - it just can’t unpack it, because the file is password protected. Adding a +, or whatever other sign into some configuration certainly won’t change anything about it.
The name of password-protected files is also not changed in any way.

I don’t know if the + sign has any special meaning in NAV for Exchange configuration (such as blocking given file types completely, without checking anything else), but in general I’d suggest to ignore the message.