@TS-DELL 

@DELL-Cares 

@DELL-Chris M 

CT2K16G48C40U5 Crucial 32GB Kit (2 X 16GB)
PC5-38400 DDR5-4800MHz non ECC
Unbuffered CL40 1.1V Single Rank Memory

May not be certified to run in your Dell.

Even crucial will NOT RMA ram that is not validated by their configuration manager to work on your system.

The issue is that on the Crucial site, the configurator offers that kit for a Precision 3660. 
So either the configurator is wrong, or the kit has a module faulty inside

I've choosen this ram because crucial site says it's compatible

https://www.crucial.com/compatible-upgrade-for/dell/precision-3660-tower

@TS-DELL , try one module at a time as the only one installed in dimm slot 0... with a bit of luck one will allow to boot (this to check if one is faulty)

I tried each single module, but no one worked.

What to do, will crucial replace the kit with a working one? On crucial site the kit is named as compatible...

It's very annoying.

I think Dell will not do anything about it. I sure they say'll "buy our original ram".

At this point I would say yes. Give them the computer model and the part number of the kit that their own site offered you as compatible, and let them fix the issue

FYI; I was in contact with crucial support. They gave some tips to fix the problem (cmos reset) but it doesn't work. They checked the compatibility again and confirmed that it should work. Now I can send a RMA request to crucial. But before this I'll try to replace the kit by vendor.

Uhm, i don't think the kit of the OP and yours , @MWC Tech , are ecc . In any case the i7-12700k should work with either ecc or non ecc modules, and the brochure of the 3660 states the option of using either ecc or non ecc ram.

Out of curiosity, could both of you write the p/n (dell and oem) written on the sticks installed at factory ?

I have the same issue. But with the 32GB Sticks by Crucial 32GB DDR5-4800 UDIMM
CT32G48C40U5. Not sure if this has something to do with ECC?

Thanks, so it's a normal udimm NOT ecc. At this point it's a bit puzzling

DDR5 16GB(density) x8 (organization), 4800 40-39-39 (speed) , UDIMM, 16Gb (die density), MPS 5100 (PMIC aka power management integrated circuit)

The kit you bought matches it, even if they give a bit less low level specs so I've no clue what PMIC is using. But density, organization speed , latency , should be a match