This came up a few years ago and I didn't make a note of it at the time so I decided that this time I'd post the solution to Thwack so that I can find it more easily next time. Kudos to Chris Foley for the research he did to get this info for me.
The issue is that the Config Download tool keeps a list of IP addresses for devices whose configs I've downloaded before. This is handy because I can just hit each IP on the list and if I'm careful to not skip a line, I've got all my configs without having to type the IP and/or fat finger an entry.
The challenge is that sometimes a piece of hardware leaves the network. We had a switch set up in a conference room long term so I was regularly backing up its config. But when that project was at an end, I took the switch back. Now I've got an IP address stored in the Config Download tool that no longer exists on my network.
The solution is to delete the file where these IPs are stored. Chris points out that while it's possible to edit this file, there are some relational tables in there that could break if the file is edited incorrectly. So I made a backup of the original file then renamed that original file so that the software couldn't find it.. The next time I ran the Config Download tool the only IP that was stored was the last device that I accessed. The remainder of the list was gone, just like I had hoped.
The file can be found in the following locations:
C:\documents and settings\all users\application data\solarwinds\SharedCredentialsDatabase.mdb
Kudos and my thanks to Chris Foley for this info,