If Dropbox finds files in its folder that aren't owned by your user, it will try to run a script with super user privileges to change the ownership of those files to your user.
If you click Cancel, Dropbox won't run the script but you'll continue to get the popup because it will continue to detect the problem.
If you're able to confirm it is Dropbox causing the popup, then the solution is to enter your credentials and let it run the script. More details below.
I just got this same popup, but not at login:
I expanded Details and then I searched the web for
org.freedesktop.policykit.exec which pointed me to
Looking through my processes for
pkexec, I found:
$ ps -ef | grep [p]kexec
root 11040 26447 0 14:16 ? 00:00:00 pkexec /bin/sh /tmp/tmp5vgk_93m
So of course I was curious what that file was doing:
$ cat /tmp/tmp5vgk_93m
chown -h -R 1234 "/home/user/Dropbox"
chmod -R u+rwX "/home/user/Dropbox"
It looks like Dropbox must have detected that there are some files in my Dropbox folder that aren't owned by my current user.
find to confirm:
$ find ~/Dropbox \! -uid 1234
Sure enough, looks like some of the files aren't owned by my user:
$ ls -l /home/user/Dropbox/Projects/picodrive/cpu/fame/famec.o
-rw-r--r-- 1 root root 1346564 Dec 18 13:33 /home/user/Dropbox/Projects/picodrive/cpu/fame/famec.o
In theory Dropbox shouldn't have any problem syncing that file since it's world-readable, but at least what they're doing makes sense.
I also noticed that while Dropbox was syncing I saw errors like this, which seems to be an additional confirmation:
Can't sync "famec.o" (access denied)
Last but not least, I'm going to put this here because I had trouble searching the web for this problem:
Authentication is needed to run `/bin/sh' as the super user