The Tascam US-1641 suddenly developed a strange illness - the digital in (SPDIF) was not working. It happened when I tried a different "boot order" instead of my usual Tascam->Yamaha AW4416->Laptop order. Tried many things including changing the cable, checking settings for SPDIF/AES etc, no go.
Finally, today, tried connecting the digital out from the Tascam to the Yamaha's digital in - then the Yamaha started complaining, Dig-St-in Sync Error! Aha! So changed the setting in Clock settings in the Tascam's control panel from "Internal" to "Automatic" - then tried digital input - working! Again closed and opened Cubase, the setting was reverting to the non-working "Internal" setting. Rooting through the help file, went to Transport -> Sync Setup and changed the sync from "Internal" to "ASIO device". Now works fine even after reboots. But the power-up order seems to be important. Probably booting up the computer without the Tascam being powered on changes the Sync option to internal in Cubase.