Based on this thread:
It is likely that OpenCV is an active plugin in the project, and you’ll be able to see the engine having numpy conflict errors at startup in the output log (if you have not run the fix that rlopezll mentioned). I was running in the Metahumans sample project which automatically had this active.
It’s also probably why the Hair Card Generator just works on a new project.
I don’t know if this is patched in 5.7, based on the forum reply I can only guess it is. I am on 5.6.1.
However, at the time I didn’t know this, and ran the commands rlopezll mentioned, which eventually led to a lot of messy errors. To fix this, I first deleted the Python3 folder in:
Epic Games\UE_5.6\Engine\Binaries\ThirdParty\Python3
And then ran Verify on the Unreal installation in the launcher, which gives a fresh copy of Python. (If you start up Unreal now with the OpenCV and Python Foundation plugins enabled you can now see the numpy conflict as linked in the above thread)
Then, go to Epic Games\UE_5.6\Engine\Plugins\Runtime\OpenCV\OpenCV.uplugin
and edit it with a text editor. Change the numpy version to “numpy==1.26.4”, deleting the hashes. The Python Foundation package will handle it.
I could run the Hair Generator just fine after that. Hope it helps.