Version 3.16.3 under Lubuntu 15.10 "Thumbnail Cache" problem.

This issue is new to me. I have used Nemo for a while now, and never encountered this issue.

Very often, when I cut-and-paste multiple photos from one tab to another, I will get a yellow-bar error message:

"A problem has been detected with your thumbnail cache. Fixing it will require administrative privileges. [Fix Now] [Dismiss]"
(see first screenshot)

I do not know what is meant by a problem with my thumbnail cache - what kind of problem?

Why would fixing it require administrative privileges?

I note that, unlike with terminal sudo usage, I must re-enter my password every time this happens, even if the previous problem
was just seconds before (sudo in terminals lasts 15 minutes or so, doesn't it? - before one needs to re-enter a password?)

Also for some reason, not all photos that are being transferred between tabs retain their selectedness.
(see second screenshot - ALL photos in the tab were being transferred there, yet not all remain selected)

I had invoked Nemo from a terminal, so that if it crashed, I would have something to attach to bug:
https://bugs.launchpad.net/ubuntu/+source/nemo/+bug/1374651
It did not crash, but since I note the presence of references to the thumbnail cache issue in the log, I also attach it,
in case it might be useful.

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scott@scott-Asus-M2M68-AM-Plus:~$ uname -a
Linux scott-Asus-M2M68-AM-Plus 4.2.0-16-generic #19-Ubuntu SMP Thu Oct 8 14:46:51 UTC 2015 i686 athlon i686 GNU/Linux
scott@scott-Asus-M2M68-AM-Plus:~$ lsb_release -dsc
Ubuntu 15.10
wily
scott@scott-Asus-M2M68-AM-Plus:~$ echo $DESKTOP_SESSION
Lubuntu
scott@scott-Asus-M2M68-AM-Plus:~$ nemo --version
nemo 2.6.7

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: nemo 2.6.7-1
ProcVersionSignature: Ubuntu 4.2.0-16.19-generic 4.2.3
Uname: Linux 4.2.0-16-generic i686
ApportVersion: 2.19.1-0ubuntu4
Architecture: i386
CurrentDesktop: LXDE
Date: Sat Oct 31 20:41:47 2015
SourcePackage: nemo
UpgradeStatus: No upgrade log present (probably fresh install)