@Mel628 According to the XPS 8940 Setup and Specifications there are only two supported 32 GB memory configurations and none of them are 4 x 8 GB.

I saw on other forum post, that 4x8Gb will still work but at a lower Mhz, but in my case it work even boot at all.

only certain user validated ram 4x8 GB works in 8940.

Re: plus the one a got recently looked different then the previous one, is that going to be a problem

that would likely be a problem causing your issue.

For user validated 4x8Gb ram in XPS 8940:

https://www.userbenchmark.com/PCBuilder?searchTerm=XPS%208940%204x8GB

One example of successful 4x8GB is CT8G4DFRA32A.C8FP 4x8GB

CT8G4DFRA32A.C8FP

CT8G4DFRA32A is the same model number for both ram shown in OP uploaded pic.

not sure what the additional 4 letter after model number means?  Crucial does not include that on their website.  https://www.crucial.com/memory/ddr4/ct8g4dfra32a

but besides the additional 4 letter code difference, you can see the number of memory chips are different for the two ram OP has (top one has two chios on one side, bottom one has 4 chips).  It is definitely not a good idea to mix those two different rams. albeit having same model number. 

is the top ram double sided and the bottom single sided?

PS here is another crucial ram  BL8G32C16U4B.8FE 4x8GB validated by user for xps 8940

https://www.crucial.com/memory/ddr4/bl2k8g32c16u4b

Wonder if disabling Secure Boot would help?  Once it boots then turning it back on.

I'd contact Crucial support directly and ask about the different chip configs and your boot failures, especially if they have the same part numbers...

Secure Boot on/off has no impact on RAM. Secure Boot only looks to see if hardware drivers (eg GPU drivers) have been digitally signed by Microsoft or not.

Re: Attached picture below, left one is the recent one that cause the crush, right one is the one working fine.

funny that I thought left one with 4 chips (which looks more like the known good CT8G4DFRA32A.C8FP) may have a better chance to work than the right one w two chips on one side only.  Crucial ram with same model number does not seem to indicate same spec of ram physically.  that is a concerning news.

@Mel628  I'd still contact Crucial for assistance. Maybe the new one is just defective.

Wouldn't be the first time that's happened with brand new Crucial RAM. And you can also talk to them about the different number of chips on these two modules too...

I just received 2 sets of the Crucial CT2K8G4DFRA32A from Amazon today for my XPS 8940 (i5-11400).  One set has a pair of CT8G4DFRA32A.M8FR sticks and the other has a pair of CT8G4DFRA32A.C4FE sticks.  So I think I have the same memory as you.   Two have 8 chips, the other two have 4 chips.  Both pairs are listed as the same model, ordered in the same order and received in the same package.   I couldn't get the PC to boot with all four sticks installed as you would normally install them (matched pairs in matching colored slots).  But any combination of 3 sticks worked.  Any three Crucial sticks, or two Crucial's and the stock Samsung 8GB stick worked.  But no combination of 4 would boot up.  Eventually I found another forum post that mentioned mismatching memory in the slots (thanks to whoever it was that posted that).  So I put matched pairs in opposite colored slots next to each other.   That worked.    I put the pair of 4 chip memory (C4FE) in the Black and White slots on the left and the 8 chip memory (M8FR) in the Black and White slots on the right.  It gave me the "Alert! The amount of system memory has changed" screen, then booted right up.   CPU-Z currently shows a total of 32GB running as dual channel at 1463.2 MHz.  Dual channel 1463.2, would be almost 2933, so I think that would mean it's running at the full speed it's capable of 2933 MHz.    It's seems to be running stable so far through all the tests I've put it through.   

I have been fighting boot issues with ram also. oe dell ram was 2 sticks 8GB. I added 2 more Dell sticks of 8GB each and no boot. after reading these posts, I installed oe ram sticks in 1/2 positions, and the additional Dell ram sticks in 3/4 positions, and  "Alert! The amount of system memory has changed" screen appeared, system booted on continue.  3 hours wasted however....all tests with CPU-z show good.

This is the first time I will interact with the Dell support forum and last time I will interact with Dell (my account was created in 2005!)

I haven't bought much Dell HW through these years, and never had to interact with Dell equipment or it's support much.

But after this experience, it will be the last time I will interact (and of course purchase) Dell anything.

So I went thru a 2 hr ordeal with tech support to explain to them that no matter which sticks, in which order, on whichever slots, the machine (XPS 8940) would not boot up. ANY 3 sticks, fine. After having them ask me again things I've already mentioned I had done (update the BIOS, clear the BIOS, try to plug in a different stick), the McDonald's level brain on the other side agreed to send a tech support to my location with a new motherboard.

New motherboard did not solve the problem. I suggested trying the PSU next, they suggested I send it in to the depot. I paid for at home service, so no waiting on the Depot for me.

Finally gave up and asked them that if they didn't knock some money off the purchase price I'd be returning the machine. They (this time the reasonably intelligent agent) said the most they could refund was 10%. So I took the 10% (I had asked for 40%) and called it a day.  never buying Dell again.

After I decided to keep this machine (kind of  build, BIOS is very restrictive) despite the mishaps I came across this thread and I can confirm that: placing matching sticks in opposite color rails, next to each other, allowed me to boot (in my case 2x16GB and 2x8GB, machine came with a single 8GB).
Dumb as Styrofoam support didn't suggest trying this, and I didn't think to do so until this thread, because we all know you usually have matching modules on same color for dual channel operation.

Well, now the BIOS still says it is in Dual Channel mode, and so does Linux (Interleaved Depth = 2), despite going against the common standard. IDK if one 16GB module is in dual channel with the other 16GB, or 8GB of each 16GB module is in dual channel with 8GB of the matching color stick (this is possible on Intel, not AMD AFAIK).

Either way, I don't care, I am done with Dell forever. Best of luck to you guys.

@DellDeleted  You should have come to the forum first!  There multiple forum threads all saying the same thing about putting matched pairs of RAM modules in slots with mis-matched retention clips.

We've asked but haven't gotten any explanation since this is different than the "normal" RAM configuration. The XPS 8950 may also do things this way too. So this may be the "new normal"...

Thanks RoHe, it would have probably save some time but in a way I am glad I've had a chance to realize how bad Dell support is. Not that I need support often, if ever, I knew exactly what to try next (MB replacement), but having to spend 2 hrs getting the as good as a bot human on the other side to finally send someone here is enough for me to never want to deal with Dell again.

Worse, as you mentioned, there are plenty of reports showing that would be the solution, but they didn't suggest that. nor did the onsite tech (a good one BTW, but how could he know).

That's why we are here, volunteering our time ... day-in and day-out. Some strange things come up with these nicer computers sometimes.

Running 4 DIMMs has always been hard. Not just with Dell, but also Alienwares and even HP's (running these similar motherboards). But it's really just because it's a "fully loaded" ram-config and has some particular dependencies (and generally, fairly common with Win-Tel PCs).

One answer is to just run 2 (exactly matching) DIMMs (in one Memory-Bank for Dual-Channel-Mode). And then, you populate the other Memory-Bank. Memory bank slots have matching-colored hold-down clips.

Or, you can usually get 4 DIMMs to work if they have the correct "Ranking". That's what users are showing with the numbers of chips on DIMMs or DIMM-chips on only one side ... it's called Ranking. You will see it in common software tools like CPU-Z.