I have the same problem and solve it by the following approach.
choose System Preferences
select the Keyboard option
Select the Input Sources tab, and then check the box next to Automatically switch to a document’s input source option
I ran into this same problem with my son, I googled and found a post where someone suggested to check
But when I did I found that "Users" was locked. I tried to chmod the settings to 777 and was blocked, so I sudo changed the settings with:
$ sudo chmod 777 Users/
For the non-native ...
None of the previous answers worked for me but, with Mojave, this one did the trick.
Instead of doing Command+Shift+3:
In the Options, make sure "Desktop" or "Other Location" is selected
Also make sure that "Remmember Last Selection" is checked
Perform a "Capture"
Then, next time you will do Command+Shift+3, it should be working.
I had the same problem description, and I was about to start looking at the filesystem when it went away. I would hypothesize that in High Sierra, there is some improved handling of the problem.
While checking my configuration, I had switched backup choices (before I was using the hard drive, I was using a remote volume for ...