I was playing w/ the incremental backup feature and it created the first large archive with out a problem, the incremental with out an issue, but when I tried to create another incremental on the same day it didn't work. It seems that the incremental archive has its name as the date. So if two are done on the same day then it bombs since the archive already exists.
In the future a great feature to have would be the ability to merge incremental archives with the main archive at at given interval. Like after 5 incrementals the last archive would get merged with the main archive. This would keep the number of backup files down yet keep backups quick and easy.