What type of ECC memory works with Threadripper (1950X)?

Unbuffered ECC (UDIMMs) is what your 1950X expects. Registered memory (RDIMMs) won’t even fit in the slots. Threadripper’s memory controller on Zen1 was designed for unbuffered DIMMs, so grab some 3200 MHz unbuffered ECC sticks and enable it in BIOS.