Unreal Engine 4.17

I don’t know anything about Unreal Engine but I can answer this:

How is that possible to downgrade Xcode

You can get old versions of Xcode from the Downloads area on the developer web site. Scroll all the way to the bottom and then click See more downloads to get to the archive.

Having said that, it seems like Unreal Engine should be supporting modern versions of Xcode, so you might want to ask about that over in whatever support channel they provide.

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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

Ye thats true, I dont get it. Maybe you can help me with this, I downloaded Xcode_8.3.xip and ran the file, it created the Xcode.app file, but I get an error "Loading a plug-in failed. The plug-in or one of its prerequisite plug-ins may be missing or damaged and may need to be reinstalled" Any Ideas.

Thanks

>I downloaded Xcode_8.3.xip

Not sure it will help w/your issue, but you might want to see this:

Please update from Xcode 8.3 to Xcode 8.3.1 or later

What version of macOS did you see this on?

When do you see this message? On unpacking Xcode? On first launch? On every launch?

Did the message indicate what plug-in failed to load?

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—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

For this problem:
Xcode fails to launch with following alert:
"Loading a plug-in failed. -- The plug-in or one of its prerequisite plug-ins may be missing or damaged and may need to be reinstalled."

I found the key to a solution here:
https://developer.apple.com/forums/thread/100026

In the Xcode app directory:
/Applications/Xcode.app/Contents/Resources/Packages

there are four packages:
CoreTypes.pkg
MobileDevice.pkg
MobileDeviceDevelopment.pkg
XcodeSystemResources.pkg

I had to open and install all of these packages to fix the Xcode launch failure. Then Xcode opens normally.

How my Xcode was damaged: After a macOS update, I ran Xcode and it needed to update it's components. That failed because of low disk space (my bad) and then I was getting the "Loading a plug-in failed." alert upon failing Xcode launches.

Perhaps Xcode code resolve this issue automatically in the future?