Unreal crashes on level load with Assertion failed: !bIsAlreadyInSet error (Level.cpp, Line: 2375) due to blueprint changes

A few days ago one of my levels started crashing Unreal upon each load with this error:

Assertion failed: !bIsAlreadyInSet [File:D:\build++UE5\Sync\Engine\Source\Runtime\Engine\Private\Level.cpp] [Line: 2375]

The problem was probably related to moving blueprints used in the level when outside the level, thus links in the level were not updated.

Eventually this problem was solved by removing Content_ExternalObjects_\Maps(level in question) contents. I did not find this solution anywhere, it was a guess.

But there are still reports on “failed to load Outer on resource” for two affected objects. I don’t know what to do with those messages. Removing contents of AppData\Local\UnrealEngine folders, as many users recommend with Outer errors, did not help. So I guess I’m stuck with those errors for now.

So, the questions are:

  1. how to avoid this kind of error completely (and should Unreal fix the Line: 2375 Level.cpp error – this function is supposed to rename duplicate folders, and crash seems to be an unexpected response).

  2. how to remove the failed to load Outer errors.

Hey @zalessky! Welcome to the Forums!

First and foremost, have you fixed up your redirectors in Engine? You can do so by right clicking your content folder and selecting the option here:

Also, have you looking inside of the blueprints in question to make sure no references are broken inside of them after moving?

Any additional information you can provide will be a big help in solving your problem!

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