So yeah. Here's the crash report:
https://bytebin.lucko.me/uw34XrKDcj

Without it, everything works perfectly fine.

Mixin issue, needs to be fixed by Optifabric, so keep this open for now. Most likely not caused by Iceberg itself, but the way Optifabric maps the Optifine changes.

can confirm, not an iceberg issue and needs to be fixed by optifabric

Hey guys, author of Iceberg here. I was able to implement a workaround in the latest release of Iceberg (1.1.13) that prevents this crash. It's not ideal, but it works!

Hey guys, author of Iceberg here. I was able to implement a workaround in the latest release of Iceberg (1.1.13) that prevents this crash. It's not ideal, but it works!

Nice, thanks for the update! Hopefully it can be fixed by Optifabric in future, as it’s most likely related to a Mixin or other.