I managed to get contact information for Nuvoton FAE, but no response yet.
Nuvoton has a technical forum but it is currently closed, you cannot register.
Nuvoton seemed to be quite a well-known brand and I was hoping for better support.
But as you can see, it's better to stick to popular brands (NXP, STM, Microchip....).
The NUC980 seemed like a good choice because of its small footprint (TQFP64) and plenty of RAM 32/64MB.
If my problem is not solved quickly, I will probably change the chip to STM32H7xx and external Dual QSPI PSRAM.
BR JarekC