MarcG

Great! Not sure why having them together is not working on 10.13, but what matters is that it’s now working for you.
 
Marking as solved. Enjoy! And welcome to the forums.

just for curiosity and trying to understand,
 
could you try this?
 
snapshot2
 
 
 

@MarcG Seems like the utility is still the same. Since your error is somewhat bizarre, this looks like an oddity in your personal setup. Just to get more data, what’s the result of: /usr/sbin/screencapture -i -P '/tmp/test.png'?

Seriously impressed. What an awesome forum.
 
Many thanks!

screencapture, which comes with macOS, is surprisingly capable (man screencapture) and can do it in a single command:
screencapture -iP "$(mktemp).png" i is to capture interactively (i.e., the same as ⌘⇧4). P is to open the image in Preview after it’s done. "$(mktemp).png" is a temporary file to store the screenshot, which screencapture needs.  
Add that in a Run Script Action connect by a Hotkey Trigger, and you’re set. Here’s a ready-made Workflow.