Question about Gigabyte X870E AORUS ELITE WIFI7

Hello together I have just put together a new setup (all freshly ordered components Dec. 2024) and the problem is that the system does not boot with 2 RAM modules -> Error Code C5. I have tested both modules independently and individually in several slots (recommended according to Gigabyte are slots DDR5_A2+ DDR5_B2) Everything works normally when only 1 bar is inserted. As soon as the second one is added, the start fails. The modules used are also in the compatibility list of the board. The system is set up as follows: Mainboard: Gigabyte X870E AORUS ELITE WIFI7 CPU: AMD Ryzen 9800X3D (AM5) PSU: be quiet! Straight Power 12 Platinum (850W) RAM: 2 x 32GB Kingston Fury Beast DDR5 6000MHz (CL30) GPU: Sapphire Nitro+ Vapor-X 7900XTX The BIOS is up to date (version F3h - 03 Dec 2024). That one of the bars is defective should be ruled out, as they work individually, right? Does anyone know the problem or does anyone know what else I can try? Does something have to be specially set in the BIOS?