Mojave quicklook stuck at loading
A combination of the answers in here + killing all QuickLook-related processes fixed it for me. I have been experiencing this ever since the Mojave update. No reboot required.

qlmanage -r commands always got stuck for me, too.

Fix TL;DR version:


Delete DropboxQL.qlgenerator from ~/Library/QuickLook (Credit to suntrop)
Restart Finder
sudo update_dyld_shared_cache -force (Credit to Dishoperator)
Kill all QuickLook-related processes
Restart Finder


Fix detailed how-to version follows

For those not very familiar with what the above means.

To get QuickLook working when previewing files with spacebar:


Delete DropboxQL.qlgenerator in ~/Library/QuickLook, as per suntrop's suggestion.


Open Finder
Press ⌘+G
~/Library/QuickLook
Look for a file named DropboxQL.qlgenerator, it will probably be the only one
Delete it, and then delete it from Trash

Restart Finder:


Press ⌘ ⌥ ␛ (Command + Option + Escape)
Find Finder
Click Relaunch



To get thumbnails to work:


Run the Terminal command Dishoperator suggested:
sudo update_dyld_shared_cache -force 


It doesn't do any harm, it just updates the kernel caches
Open Terminal
Paste sudo update_dyld_shared_cache -force
Type in your password (it will not appear as you're typing)
Press Return
It will take ~30s to a couple minutes depending on your machine

Open up Activity Monitor (⌘+Spacebar, type in Activity Monitor)
Order by Process Name
Force quit quicklookd


Click on quicklookd
Press the X stop-like button in the top left 
Press Force quit

Force quit all instances of QuickLookSatellite
Keep force-quitting until none of them are left


Make sure you order processes by name to see them all at once

Restart Finder (see point 2. Restart Finder: again if you forgot how)


Keep having problems?

Since this is now working for me, but in case it still doesn't for you, here are some apps that would normally be suspects but are NOT the cause. This should narrow-down your search:


Dropbox (was a problem)
Sketch 52.2
Principle
CraftManager
Photoshop CS6
Illustrator CS6
Bear
Highland 2
Kindle
Atom, PhpStorm, Sublime Text, Caret
p4merge
Slack
Screenhero (don't know why I still keep it)
SourceTree
Chrome, Safari, FireFox
Franz
1Password
Spotify
Shazam
TeamViewer
Steam
Flux (don't know why I still keep this one either)
Microsoft OneNote
Bartender 3
PSequel
Postico
GoToMeeting
Kaleidoscope
MindNode 5
Podio
Fluid UI
Gliffy Diagrams
Conceptboard
Moqups
Wireframe.cc
Graph.tk
SketchUp, Layout, StyleBuilder
Google Keep
VLC
Trello
Asana
LiveMinutes
Zoom
Amphetamine


And remember to clean up useless apps from your laptop. Which I clearly don't.