Unreal Egnine 4, all of them, stopped working - Failed to launch editor

Development Getting Started & Setup
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So, I’ve been working in UE 4.27 and 4.26, but I have also older versions installed.
Been all working great for years. Today morning opened the projects, everything worked.
Didn’t restart PC, left it idling for an hour.
Came back, tried to launch 4.27 project - got an error:

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When I launch the engine from the Launcher, and then browse to the projet, all works great.
Now, I cannot generate project files either. It worked last week. I didn’t update windows, did nothing.
The error was first due to UnrealBuildTool being in wrong folder, ChatGPT helped me with that, so I built that and now I get an error when trying to Generate Project Files:

UnrealBuildTool.Main: ERROR: Unhandled exception: System.IO.DirectoryNotFoundException: Nie można odnaleźć części ścieżki „D:\Epic Games\UE_4.27\Engine\Engine\Source”.
UnrealBuildTool.Main: w System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
UnrealBuildTool.Main: w System.IO.Directory.SetCurrentDirectory(String path)
UnrealBuildTool.Main: w UnrealBuildTool.UnrealBuildTool.Main(String ArgumentsArray)
UnrealBuildTool.Main: DirectoryNotFoundException: Nie można odnaleźć części ścieżki „D:\Epic Games\UE_4.27\Engine\Engine\Source”.
UnrealBuildTool.Main: w System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
UnrealBuildTool.Main: w System.IO.Directory.SetCurrentDirectory(String path)
UnrealBuildTool.Main: w UnrealBuildTool.UnrealBuildTool.Main(String ArgumentsArray)

It doubles the “Engine” folder for some reason.

I tried reinstalling Unreal Engine 4.27, didn’t help.
Unreal Engine 5, both 5.5 and 5.6 works great.
What happened? How do I fix this? What the hell is going on? I did nothing wrong…

Help.

5 Likes
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Same thing here. One of our team members reported this issue yesterday on his project. I don’t know if its related but I coincidentally noticed an epic game launcher update sometime that same day, it continued to work fine for me that day, but upon restarting my machine this morning, None of my project files will open directly either without giving the same Error.

I’ve already tried verifying the installation. My next thought is to re-install unreal, the launcher or both, but I’d like to avoid that headache if I can.

1 Like
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Well, I’ll start with the good here (kind of):

  1. You aren’t the only one having this issue. I’m aware of at least 3? Individuals that also have this issue starting yesterday.
  2. I unfortunately don’t have a solution for you, but that said, I’ll keep tabs on this thread in case you or them happen to find a solution. If they should happen to find one, I’ll be sure to reply

Now for the bad:

  1. I believe the individuals in question have been looking into this fairly heavily and have yet to find any leads. That doesn’t mean all hope is lost, but I am surprised there’s nothing yet as they’re fairly competent users…
  2. Seeing as it’s a non-isolated issue and it all happened at the same time time, I’d unfortunately have to think it may be related to Epic games somehow and that this is out of our hands…
1 Like
4

Same here. I have both UE4 and UE5 installed, but my primary IDE is UE4, and all of the sudden all UE4 projects cannot be launched and this error message pops up!

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Not a fix, but a workaround that seems to have got things working again was re-installing the epic game launcher. Initially I used the most recent installer EpicInstaller-19.0.0 but that one was unable to download any plugins (kept stalling at 0% with unknown error) so I used an older installer we still had ‘EpicInstaller-14.6.2‘

After it was done installing and updating it shows as 1.3.67.0

When I first opened it, it did tell me there was a newer version and asked me to update, but this time I said no. I was able to re-install ue4.27.2 download and install the plugins I needed and open projects directly again.

My guess is something in the updated version is broken as far as unreal 4 projects and hopefully they’ll resolve it soon.

2 Likes
6

Hello! The same thing happened to me today and I fixed it by installing a previous version of Epic Games Launcher, version 18.8.1 specifically. I recommend uninstalling Epic Games Launcher and Epic Online Services and then installing 18.8.1 from here: https://epic-games-launcher.en.softonic.com/versions

During installation it will download the latest update (which caused the issue) but you can decline installing it later by pressing ‘Cancel’.

1 Like
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Yep, this seems to have fixed the issue. Thanks!

8

I was trying to reinstall 5.5.4. This thread provided the solution. Thank you!

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I also encountered this problem. The new version of the launcher makes it impossible to download version 4.27. Additionally, when trying to regenerate the project files, the following error occurs:
“Missing UE_4.27/Engine/Binaries/DotNET/UnrealBuildTool/UnrealBuildTool.exe after build”

I also tried it on a secondary PC, and once the launcher is updated, everything breaks.

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A workaround shouldn’t have this marked as resolved. When is epic going to fix this? Why wasn’t the launcher update rolled back as a temporary fix by them?

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This issue ISNT resolved, its still happening. Leave it up to someone to mark a Existing issue as resolved when there isn’t any practical fix for the issue.

Contacting Support on this gets you no where either. they just run you through basic troubleshooting steps…

  1. disable all background apps.
  2. verify engine version.
  3. Verify Launcher.
  4. Run launcher as Admin.
  5. Perform a clean install of launcher - which also causes you to reinstall All engine versions you had installed.
  6. If all this fails, please email your launcher log files to a support ticket.

there is nothing stored in any of the log files that i can see that even addresses the error box popping up going “cant launch editor” ..

Does Epic even run their own software? you’d think it would be a simple issue to reproduce considering it continues to happen even on a clean install.

Thanks Epic, Now I can’t work on my projects because of your screwup. Guess i should of stayed with Unity.

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Thanks for the tip! I tried something similar and it worked on my side too — rolling back the Epic Games Launcher really does help. Uninstalling both the launcher and Epic Online Services first is a solid move, then reinstalling the older 18.8.1 build and canceling the update stops the issue from coming back.

Appreciate you sharing this fix — super helpful for anyone stuck, especially for people juggling work like ranqeo projects or seo services on the side!