Quicklook not working
Try the following:
- Open the Terminal application (/Applications/Utilities/Terminal.app)
- Type
qlmanage -r
and hit enter - Type
qlmanage -r cache
and hit enter - Close Terminal and restart your system
This should force QuickLook to reload the generator lists and reset the disk cache.
Source: https://www.macissues.com/2014/03/22/fix-quicklook-errors-in-os-x/