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.
Good luck!
Open Task Manager, go to details, there will be a “UE4 crash hander” kind of program running. close it (Make sure you aren’t closing the actual editor) That helped me save map-built data that wasn’t being saved before.
You are a life saver! Had the same problem, easy fix is to close the duplicate editor.
Just popping by in 2021 in UE5 to say thanks! 
haha me 2
fast forward 8 years and this still gets people lol
dude. i can’t believe the same thing happened to me!!! thank you so much i was losing my mind. i even shut down/relaunched the 2nd editor over and over again!!!
That was exactly what I needed. Thanks, man!
Thanks!
HAHAHAHAHHA same, i feel stupid
" mmmh this post is from 2014. Let’s see if it’s still relevant. "
It is. 
I’ve been looking for the second instance of Unreal being open and even tried starting from a computer restart. It is weird that I can not save Materials but saving everything else is fine. If anyone has any other suggestions, it would be appreciated. 
It’s so sad that this hasn’t been upvoted at least once in 7 years…
This should be a built-in feature, it should at least show you a simple prompt like: “There’s an instance of this project already running, do you want to continue?”
This was me! Thank you for that
Same today 
Hey i had the same issue just now. I recently updated to Windows 11 an have never recieved this message before. It was weird because i was saving my project not 2 minutes before. I opened a new hud and tried to save, didnt matter what i tried to save after it all broke. I fixed this by restarting the computer. Hope this helps anyone with the same issue!
You probably had the same project opened twice (like many of us in this thread)
I was sick of this happening (and of losing my work) so I made a plugin to prevent it.
Another way: You can copy blueprint elements into a different blueprint type even if the destination blueprint doesn’t support one of the elements copied without an error message. (5.1) The blueprint asset won’t save without removing those elements, but this is the error message without any context about why.
Lol had the same problem. Didn’t even know I clicked twice on my project.
Such an old forum with a common sense and easy fix, yet we are still here looking for an answer ><
Just had this issue today as well , glad this form lives