Thank you all for your comments. I have contacted DVDFab customer service and this is their reply:
DVDFab UHD programs only support the following drives:
UHD Official Drives:
LG WH16NS60(Internal, Firmware: 1.00) (Recommended)
UHD Official Drives:
Archgon MD-8107S-U3-UHD BU40N (external, USB) Tested
LG WH16NS60 (Internal, Firmware: 1.00)
Buffalo BRUHD-PU3 (External, USB, Firmware: BU10)
LG BU40N (Internal Slim/Laptop, Firmware: FR07, TST0, BU10, CA03, A100)
LG BU50N (Internal Slim/Laptop, Firmware: GE02)
UHD Friendly Drives:
LG BE16NU50 (External, USB, Firmware: 1.01) (Recommended)
LG WH16NS40 (Internal, SVC Code: NS50)
LG BH16NS40 (Internal, SVC Code: NS50)
ASUS BW-12B1ST (Internal, Manufactured 2015+)
ASUS BC-12D2HT (Internal, Manufactured 2015+, Firmware: 3.00)
ASUS BW-16D1HT a (Internal, Manufactured 2015+, Firmware: 3.01)
LG BH16NS55 (Internal, Firmware: 1.00~1.02)
LG WH16NS58 (Internal, Firmware: TST5)
ASUS BW-16D1H-U A201 (External, USB, Manufactured 2015+, Firmware: A201)
LG BU30N (Internal Slim/Laptop, Firmware: 0217, 0245)
If your drive is already with the latest firmware version that is not backup-friendly, you can use DVDFab UHD Drive Tool to downgrade the firmware to the previous one. Check out the supported list below:
UHD friendly drives:
LG WH14NS40 (internal, SVC code: NS50) Tested
LG WH16NS40 (internal, SVC code: NS50) Tested
LG BH16NS40 (internal, SVC code: NS50) Tested
LG UH12NS40 (internal) Tested
LG BH16NS55 (internal) Tested
LG BH14NS58 (internal) Tested
LG BH16NS58 (internal) Tested
LG WH16NS58 (internal) Tested
LG BE16NU50 (external, USB) Tested
ASUS BC-12B1ST a/b (internal, manufactured 2015+) Tested for a
ASUS BC-12D2HT (internal, manufactured 2015+) Tested
ASUS BW-16D1HT a/b (internal, manufactured 2015+) Tested
ASUS BW-16D1H-U (external, USB, Firmware version started with letter A, manufactured 2015+)
UHD official drives:
LG WH16NS60 (internal, recommended) Tested
Note: This list will be constantly updated as we support more drives in the days to come. For more information please check this link: https://www.dvdfab.cn/4k-uhd-drives.htm?trackID=productpage
Perhaps tbirdvet does not have one of the supported drives for reading 4K UHD discs?
I'll try with one of the supported / recommended external drives and will let you know what happens. Thanks.
BTW, "Simple mode" skips the disc menu and plays the main video feature. The other videos can be accessed via the player's menu, or so it seems.