The crash seems to occur due to the way immersive engineering handles it's scaffold blocks [which share their block ID with the metal fences].
crash-2021-04-18_15.14.54-client.txt

When I tested it, there was no error.

Could you take a video of the error situation and upload it? Furthermore, please explain in more detail.

Whenever I attempt to climb Steel Scaffold [or aluminum]. Via the ledge climb mechanic, The game crashes. Here is a video demonstrating this.
https://user-images.githubusercontent.com/20866677/116797108-a043f280-aaa7-11eb-91dd-4ae5b0ca4a32.mp4

This is tested with the latest version of SmartMovingReboot and Immersive Engineering latest 1.12 version. I discovered this issue in my modpack, but have tested it with only the two relevant mods [this should be visible at the first few frames of the video]. I am running on Windows 10 if that helps.

Ok, I checked.