The error "inconsistent assumptions over interface" has always been due to
a build artifact not being updated after upgrading some library. Rebuilding
one's files from scratch always solves the problem. However, I'm in a
situation where I've recompiled everything from scratch many times, but I
reproducibly get the error below. Does anyone know what else I should look
for?

It's Eliom related, though I don't know why that would matter.

# Error: The files ~/.opam/4.02.1/lib/eliom/client/eliom_pervasives.cmi
#        and ../_server/foo.cmi make inconsistent assumptions
#        over interface Eliommod_parameters
# *** omake: targets were not rebuilt because of errors:
#    _build/lib/_client/foo.cmi
#       depends on: _build/lib/foo.eliom
#    _build/lib/_client/foo.cmo
#       depends on: _build/lib/foo.eliom