Fabric, Log: https://mclo.gs/7MShham
Removing Custom Player Models fixes the crash from placed skeleton skulls.

Update iris

Sadly I can't. Immersive portals will not work past 1.4.5. Is there any other way this could be patched for people in my situation?

Open the CPM mod jar with something like WinRAR or 7zip.
Edit cpm.mixins.compat.json, delete the line BlockEntityRenderDispatcherMixin_Iris in it.