I'm trying to run elog behind haproxy, but get the error "Invalid Content-Length in header" on posting.

As stated in the manual, haproxy rewrites all headers to lower case.

elogd parses the content-length header case sensitive which is against the HTTP RFC. This might also apply to other headers that get parsed.

For now I'm using the workaround from the manual:

global
  h1-case-adjust content-length Content-Length
  h1-case-adjust content-type Content-Type

backend elog
  option h1-case-adjust-bogus-server
  server elog 127.0.0.1:8080

But as the manual states, this should not be  used as a permanent solution.