So once again I fixed my issue myself by pure persistence.
To fix this I had to move the InstancedFoliageActor itself a couple meters up so everything is off the landscape.
Then into the Foliage Tools, I select all foliage and pressed END and now it didn’t crash !
A couple foliage actors were floating above the trees now cause they were colliding with the trees for whatever reason but those were easy to fix.