I've checked through Quicklook a few broken jpegs, I noticed after a while it started showing them as solid green image, the more I went on the more often it showed the green image. Now every jpeg I try to preview looks like this
I've read there was a similar bug on 10.11 (https://www.cultofmac.com/400506/large-jpegs-lead-to-green-screen-of-death-on-el-capitan/).
The image shown in the above screenshot is this:
To trouble shoot further you can:
—Try a SafeBoot https://support.apple.com/en-us/HT201262
Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc. Login and test. Reboot and test as caches get rebuilt.
In Safe mode third party system modifications and system accelerations are disabled, it removes malware, etc hampering smooth operation, however a reboot will put it back to normal mode.
This test will tell you if third party interference; extensions etc are not loaded in safe boot mode.
—Test issue in another user (or guest user) account https://support.apple.com/en-us/HT204443
This will tell you if it a universal issue or isolated to your user/admin account.
Sorry, forgot to add details.
I'm using macOS 10.15.4, fully updated, on a mid-2015 2,2GHz i7 16 GB retina 15" MacBook Pro with all original parts. The jpeg files are on an external USB drive but the screenshot is taken with a in image on the Mac's SSD
Restarting the Mac it starts working again, unless I go back to checking those pictures, in that case the dreaded solid green preview comes back.
I have an university exam in 2 days, I'll tell you about how and if the same thing happens in safe boot mode after that.
I'll let you know, I don't know if I have time for safe boot if it's gonna take a while to boot (I'm used to cmd + S boot, but I don't know it safe mode in Catalina is actually the same.
The external HD is ExFAT formatted, but the same thing happens with files saved on the Mac APFS formatted SSD. It has not been edited at all, the file I showed you was perfectly fine (jpeginfo command said it was ok, has always worked and was created by importing from the camera to iPhoto in 2004, so the maker software is QuickTime 6.5.2 on Mac OS X 10.3.6).
Here is the exiftool output for the file: https://gist.github.com/giovariot/a19822eed2c390d1ebfefedebc63e677
How is the external HD formatted? Where are the jpeg files from? Are they standard jpegs? Have they been edited in a 3rd party editor. If so what editor?
Did the Safe Boot fix it?
You do not say what exact Mac /model /year /size /retina /TouchBar is this
or
what exact macOS you are currently running?
Just to make sure: did you restart your mac?
Quicklook shows every JPG as a solid green image after opening a few corrupted jpegs
