Hello Ludovic, Jan,
About the bootstrap ext2fs.static server, it seems that it is also required to create a link in /libexec, attached a patch.
and now it shows another error:
--
Hurd server bootstrap: ext2fs[device:hd0s1] exec startup proc auth.
/libexec/console-run: /dev/console: Permission denied
/libexec/console-run: cannot execute /libexec/runsystem: Exec format error
--
I'll keep checking ..