Off memory, to do odd CAS settings isnt hard or a bug, find the Gear Down Mode and disabled it (in the manually set timings)
If this does not fix your issue, then post back and I'll refresh my memory / locate the correct setting to change however I'm fairly sure this is the issue fixed.
It did the trick. C15 is running now @1800 and 1833. 1866 results in a safe boot.
Had to up to 1,435V though for stability.
Time to fine-tune and search the sweet spot.
Good to hear that did the trick OP.
For me with a 3900x, 1800 (3600) is the sweet spot. I tested from DDR3200 to 4400 on samsung b-die before settling.