When I run Update Manager I get this error:
This is what the error box says:
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: http://linux.dropbox.com lucid Release: The following signatures were invalid: NODATA 1 NODATA 2
W: Failed to fetch http://linux.dropbox.com/ubuntu/dists/lucid/Release
W: Some index files failed to download, they have been ignored, or old ones used instead.