iMac crashing after memory upgrade, .AssetData issue

As mentioned,Mac’s are very RAM picky.

In addition to having a bad or a loose module, the RAM could be incompatible.

I would proceed with your 2 module testing and then if that doesn’t help. I would reinstall the factory RAM modules and test.

Then I would request a refund from the seller. Next I would purchase a couple of 16GB modules from OWC and add those to the factory RAM modules.

https://eshop.macsales.com/item/OWC/2666DDR4S32P/

i would return to the original RAM that came with your Mac -- make sure they are in the correct slots (slot position matter)

if those work -- swap in the two aftermarket and test (remove the good RAM)

if neither work now -- i would suspect bad install (possible conflict) or bad hardware

bad install: Erase and install -- to an external SSD -- add the suspect only -- reproduce the problem

bad hardware -- return Mac to factory original hardware -- remove all non-Apple mice, keyboards, hubs, cables, ect...

ps: strong electronic interference -- like a microwave close by -- can also cause issues

I've never seen the ".AssetData" item mentioned before, but I have seen the "Creedence Cryptex" disk image on macOS 15.x Sequoia automatically mounting for some unknown reason. It seems to be part of the new thinking at Apple to have system items mounted in plain view and not unmounting them when they are done. Technically that is not a problem, just an annoyance....just ignore that mounted disk image (eject it if you can....people report it cannot be ejected). You can provide Apple with product feedback for the ".AssetData" & "Creedence" disk image issue here:

Feedback - macOS - Apple

For the system crashing issue.....that may be related to your original performance issue which likely was the reason for the memory upgrade in the first place. I see in the picture of Disk Utility you provided that your system uses a Fusion Drive. More than likely the Hard Drive portion of the Fusion Drive is worn out or actually failing. You can check the health of the Hard Drive by running the third party app DriveDx (free trial period) and posting the complete text report for the Hard Drive portion here by using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar. I will be happy to look over the DriveDx health report. FYI, typically the Hard Drive will be the second "Macintosh HD" item on the left pane of Disk Utility, but double check to be sure.

People should not blindly purchase memory upgrades without first confirming more memory will help. You can easily determine this by looking at the Memory Pressure graph in Activity Monitor. Run your computer with your workloads for a day (or several days). If the memory pressure graph is in the green, then memory is not the problem. If memory pressure is in the yellow, then you may benefit from adding more memory or you could adjust the workload by closing unnecessary apps....yellow is a borderline condition. If the memory pressure graph is in the red, then you could benefit from adding more memory.

The greatest performance boost usually involves using an SSD (for an iMac, this means installing & booting from an external USB3 or Thunderbolt 3 SSD. Issues with third party installed software are also a very common source of performance issues.

macs are pretty picky with memory modules, so modules it would view as periodic defect would cause exactly what you describe

I would, and I would be sure the remove the ones in pairs in terms of dual channel pairs