I believe I have the same issue as Walton Hoops. I do not have a serial console, and all my filesystems are NILFS2, so I do not currently have an easy means to capture the crash. However, I experienced the same Kernel BUG on upgrading to kernel 5.11 from 5.10. Reverting back to 5.10 results in a clean boot. The actual kernels I use are the Ubuntu 'mainline kernels' 5.3.10-050310-generic works 5.3.11-050311-generic crashes with Kernel BUG. So I suspect a change between those two versions has triggered the problem.
