Indeed a bit strange
There are different things being kept in mind.
- The IP address for the bootloader is stored in flash - normally 192.168.2.71,

access is only possible from the same subnet e.g. 192.168.2.47
The IP can be configured and is stored in flash. Access to the bootloader is possible with a
serial cable, always.
- Login and user setting are stored on the system partition of the disk
You could disconnect the drive and reboot. Then it should be possible to log on through the web-if with default settings.
I don't think that firmware reflash will help.
You could connect your NAS with USB, remove the system partition, restart with LAN and restore a previously saved configuration.
Hope it helps,
HWguru