If all else fails to get access to the DL4100’s dashboard and access can’t be obtained using a web browser, you could try a system restore reset using the reset button on the back.
Before you do that, try simplify your computer. Turn off any 3rd party firewalls and proxies and use a web-browser with all add-ons and extensions disabled. Make sure you have Java installed. If in doubt, install it again.
Try to access the DL4100’s dashboard.
I believe (and @ERmorel may be able to confirm this) to perform a system restore reset you shut-off the NAS. Press and hold in the reset button and power-up the DL4100. After 45 seconds of holding in the reset button counting from power-up, release the reset button. Should that work you’ll then need to program the DL4100 with you desired configuration.
If that don’t work and it’s under warranty then arrange an RMA, but keep the hard drives. Number them so you know in which order to insert them into a replacement DL4100 enclosure. (I think it does not matter in which order the drives placed in the DL4100, buy always good to play safe.)