Here's how I got the error "Installation security error. Unable to Install." and solved it...
I tell the full story what I was trying to do...
I was copying contacts from Nokia 6680 to N8.
First, I tried to make a back-up with PC Suite, but it failed: The contacts would not be copied from 6680 to the computer (in a NBU file). PC Suite just gave the error "a fatal error happened during the backup operation" error code 0x80400005.
Then, I tried the Phone Switch application, which is supposed to copy contacts to N8 from another phone. I activated the Phone Switch in the N8 (applications -> tools -> phone setup -> data transfer -> another phone).
Phone Switch attempted, using Bluetooth, to install the required application also to the 6680. However, installation on the 6680 failed with the error: "Installation security error. Unable to Install."
Then I found the information on zolved.com discussions: The phone date (the 6680 phone's date) must be earlier than 12.6.2007! Then it worked!!! (I set the 6680's date to 1st March 2007, that made the trick)
Finally I was able to install the copier application and transfer my contacts. But why on earth is this crazy date setting thing needed, before I can install some application!?