This looks very strange, given your error message I agree with you, it looks like devfs is not up.
try:
grep devfs '#c/drivers'
to prove that the devfs driver is compiled in.
Interestingly your fossil conf seems to refer to
/dev/sdC0/fossil rather than /dev/fs/fossil. This would
mean fossil would start normally (not with an error)
but not use mirroring.
Perhaps this fossil.conf info is a modified version
of the config you used when you had problems?
-Steve
