Added a link for you. Alternatively you might get lucky with Pc world / curry’s type of store. I’m assuming you’re U.K. which could be an error.
Incidentally, these are the types of cables PC nerds have stashed away in abundance. If you know any, they’ll almost certainly give you one for nothing so phone a friend! 
I’m in the US, so thanks for the link. I don’t know any computer people since I’ve been retired for 18 years. They’re a whole new generation I suppose.
Keep on at Roon support tho. It’s an educated guess and I don’t know much about the nucleus except that it’s fundamentally a computer - other things I would try myself would be remove and reseat the NVME drive if there is one, and ensue that the CPU /heatsink mating is good so that overheating isn’t occurring.
Good luck, hope you get it resolved soon so you can forget about it and just enjoy your music
This might be an odd question but does it feel like it is running colder?
That case is the heatsink. The thing won’t run colder by adding RAM. So, if it feels colder, all that heat that was leaving via the case is now trapped inside. Meaning, where the components come in contact with the case to transfer heat away may no longer be where they need to be. Wild guess though.
As other have said I would certainly reseat, that is unplug everything and reattach it, to verify nothing is loose.
Also, when you say “locking up” you’ll only know this by connecting a keyboard and screen to it. Not being able to reach it from the network but being able to attempt a login is not locking up. I have seen NuC network interfaces fail. If the network interface is failing and the thing is just falling off the network then it would have zero to do with anything you did to install the RAM. It would require a new motherboard or a USB ethernet adaptor though.
EDIT: The Internet told me that my use of the word “cooler” was incorrect so I’ve fixed it. I apologize to The Internet.
Thanks for that. It’s still running now, so I have my fingers crossed. I can’t tell that it’s any cooler or warmer than before. I’m just about ready to go back in there and reinstall the new 2X8 chips.
Crapped out again. This time I restored a backup from night before last because I know it worked well yesterday.
Fingers crossed again. Still playing since I restored backup from night before last. I also put the new 2X8 RAM back in since evidently that’s not the problem. The little slot between the pins was offset to the wrong side so I had to pound it in with a hammer.
Just a little humor to ease the frustration.
I really like this album.
One wild thought is oxidation in the memory slots , maybe push it in and out a few times to get a clean patch
I had this years back, pc stops , take to menders , pc works …repeat
Eventually it worked , each car journey did something, I put it down to this.
I am not sure what contact cleaner is appropriate for PC boards , @danny may comment
It’s been playing several hours now, so maybe the backup restore from two days ago fixed something. I deleted last night’s backup so as to never use it again.
My Nucleus is working well so far this morning. It’s much snappier with 16GB of RAM. I can play a 3 minute song in 2 minutes.
I’ve been watching this thread and am so pleased that it’s worked out. I know that feeling well of trying failing to resist the urge to “tinker”, and then failing.
Glad you appear to have it sorted. Your comment reminded me of the surrrealist American comedian/actor Steven Wright. " I have a microwave fireplace. If I want to spend the evening lying in front of the open fire, it takes two minutes" 
OK, just to close this out, my Nucleus has been running 3 days with the new 2X8 RAM chips with no issues. What seems to have solved the problem was the restore of a backup from 2 days prior. My advice to everyone is get yourself a 1TB USB HDD and do automatic backups every night. I keep 30 nightly backups plus 12 monthly backups.
My old 2X4 RAM chips are available for free in the Roon “Sales and Trades” category.
Does this indicate the most recent backups were corrupted somehow? If so, I wonder why Roon didn’t report any issues when performing the most recent backups (post build 880) as it should?
Like you said, this highlights the need for keeping multiple backups.
I did not have any issues going from build 831 to 864 to 880 to 882 to 884. The only thing I can think of is something got corrupted when I upgraded from 8GB to 16GB of RAM or the night before. Or, it’s possible there was never an issue with the Nucleus and it was related to my network.
Solar flares were mentioned.
Here’s another thing I’m thinking. During the RAM upgrade, I shut down the Nucleus and unplugged it from the ethernet cable and power supply. I’m coming to believe that anytime you do that, you need to reboot your router as part of the process of plugging back in and starting up.
It could have been as simple as an iffy connection on the ram, a bit of grit in the slot or oxidisation like Mike said before. Or solar flares. Or aliens. Glad it’s working tho, now to leave well alone and enjoy! 
Common sense says if I ain’t broke, don’t fix it.
Audiophiles, engineers, and alpha nerds think: if it ain’t broke, I can fix that! 
