The situation:
Running `circleci config validate` against a GitHub Enterprise / CircleCI Server project failed with: Error: config compilation request returned an error: Not Found. The config itself was fine; the CLI (newer versions) calls a config-compilation service endpoint on the server, and the reporter's self-hosted server version did not have that endpoint exposed yet, so the request 404'd with an unhelpful message.
What actually fixes it (verified in the thread):
The cause is a version mismatch: newer CLI versions validate by calling a config compilation endpoint that older CircleCI Server releases do not expose, so the server answers Not Found. Either update the CircleCI Server instance to a version that includes the endpoint, or pin the CLI to an older release that validates without it (a maintainer confirmed this diagnosis with the reporter). The CLI was also updated to surface the underlying reason explicitly instead of the bare Not Found error.