So @lah7: this happened:
It got stuck here, it's impossible to cancel, close the window or the Boutique.
I'm not sure what to do, good thing I don't have to reboot yet. ^^
HI @ouroumov,
what about trying to install it using Synaptic and force quit the terminal/Boutique first?. 
This is what I would typically do under such circumstances:
Force/quit the window or, if the system was completely locked, then power it off with the power button.
Get back to the desktop (hopefully unproblematically) and open a terminal and issue the following:
sudo apt-get remove caja-dropbox
sudo apt-get purge caja-dropbox
sudo apt-get autoremove
sudo apt-get install -f
sudo apt-get update
sudo apt-get install caja-dropbox
Hi ouroumov
Could this of been a fluke? The Software Boutique (16.04) installed DropBox for me without issue.
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I think it's because of the crappy wifi I'm using.
Most likely connection failed after establishing connection to the dropbox website.
Quote from a website:
The default value for the “TCP Established timeout” on a Linux server is 5 days.
I wonder if it's the same for a desktop Linux machine. xD
Edit: btw the window is still stuck.
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So I forgot to update this thread.
I had to shut down eventually.
Upon reboot I ran the fixes from the software boutique.
All good.
Weird though, and apparently some other dude on IRC is now experiencing something similar:
<Dekkard> can i get a hand ?
<Dekkard> doing an upgrade and synaptic and the who thing is stalled on dropbox