SOLVED (found a reason): It seems to be, that Avast Internet Security software blocks something after the latest FW update. When turning off Avast, WD Quick view is working as it should be… Hmm…
After the latest firmware update (DL2100 → FW: 2.10.310)
WD Quick view (ver 2.4.13) and
WD Smartware pro (ver. 2.4.13)
dont recognize DL2100 anymore.
A screenshot what WD Quick view does only:
Mapped drives work fine.