Package: src:linux Followup-For: Bug #1029336 Thanks, Salvatore - I can confirm that the issue is fixed after an install of version 6.1.7-1 from 'unstable'.
The system _also_ boots fine after installing version 6.1.4-1 from 'testing'. Perhaps there was a corrupt/misconfigured initrd build on the machine before. Let me know if more details would be useful; the issue is resolved here.