Same error with me on a std. DL380G7 (P410i) ONLY with W2K3 versions. On W2K8 versions V6.40 works, but on W2K3 versions i use as a maximum V6.00.
Did you pay attention to the remark Date XX/XX stamped on the battery ??
First is Month, Second is Year, so 06/08 is juni 2008.
Sometimes you get 'new' battery's, who are in fact old.
When you replaced it, it will give you the POST error , that it's not fully loaded, but after approx. 1 hour it will be loaded.
When it says FAILED on POST, then its not going to be loaded at all: Battery or cable defective/not connected !!