I was having the same problem when I found this article:

http://support.microsoft.com/?kbid=303229

I am a backup-ignorant but it seems that when the recovery model is set to "simple", then it is not possible to backup the log db (probably because it would be useless). This is why the related scheduled job was failing while the same job for the db (and not the log) was working fine.

I think that the Enterprise Manager should check the recovery model used for the specific database and prevent users to try to backup the log while it is not possible.

Or, perhaps, I should read some books before playing with backups...

Thanks,

Paolo