Since you now have a new OS installed, I would imagine your new installation of Time Machine does not want to sync with the data file created by the old installation created under Leopard.
I would delete the sparsebundle file on your external and start Time Machine again. It should start a new backup from scratch. Even if you could continue to write to the existing backup, there would be so many system file changes, that I'd worry about the backup's integrity. Although Leopard and Snow Leopard look almost, they are completely different operating systems.
When OS 10.7 Lion comes out, once I have it up and running, I'll be deleting my Snow Leopard backup and starting over regardless of whether or not it allows my to continue to write to it.
