The underlying issue is that the unikernel has been created as "Hello", but in mollymawk everything is with a lowercase. Manually fixing the URL (from unikernel=hello to unikernel=Hello) helped me to get my hands at the unikernel.
Would be great to investigate where and why such lowercase transformations are done and whether there is anything to fix in albatross or mollymawk. Related feels the recent deployment issue of mollymawk (where a policy for robur and one for Robur existed).