I’m from 7 years into the future guys.
In case yoo’re having this error and you just have one editor open:
Check your task manager for left-over UE4 Processes.
In my case, the editor crashed, and when I reopened it, i Couldn’t save my blueprints. This was because the previous UE4Editor Process was still running. I used the Task Manager in windows (in linux use pkill) to kill the process.