Maximus XI Code RAM issues

[JanPhoenix19]

Hi,
I got a Maximus XI Code Motherboard about a year ago and used it with 16 GB (2×8) Crucial Ballistix DDR4-3600 CL16-18-18-38. Now I upgraded to 32 GB. The 3600Mhz are not availlable anymore so my local Computer Shop sold me 3200Mhz sticks. I asked if they would work with my already installed 3600Mhz sticks and they said yes. In fact, my Computer boots, but if I use more than 16 GB RAM my Computer crashes and reboots. My local Shop said, they would fix it for around 70 bucks. Does anyone know a Solution to this Problem? Debug Code F6 or Fb is shown for about 3 seconds, then the post process continues. I got the 3200 sticks both in channel A and the 3600 sticks both in B.
My CPU is a Core I9 9900K and my GPU a Rtx 3060.

[no1yak]

When will people learn not to mix Ram. If you want 32GB of ram then you buy 4x8 kit or 2x16 kit. Mixing ram is hit and miss and in your case it's miss. Adding two sticks of deferent speed ram is not good. No wonder your MB is kicking up a fuss.