diff --git a/src/grimoire/services/backup_s3.py b/src/grimoire/services/backup_s3.py
index 63324f1a..cf54bc32 100644
--- a/src/grimoire/services/backup_s3.py
+++ b/src/grimoire/services/backup_s3.py
@@ -120,6 +120,12 @@ def _says(response: httpx.Response) -> str:
         "AccessDenied": "the key isn't allowed to do that",
     }
     said = hints.get(code) or message or response.reason_phrase
+    if not code and 400 <= response.status_code < 500:
+        # S3 refuses with an <Error><Code> body. A 4xx without one came from
+        # something else at that address — most often a web server on the
+        # default port, when the S3 service listens on a port of its own.
+        said = ("the address answered, but not as S3 — check the scheme and "
+                "the port (an S3 service usually listens on its own port)")
     return f"HTTP {response.status_code}{f' {code}' if code else ''}: {said}."
 
 
diff --git a/tests/test_services_backup_s3.py b/tests/test_services_backup_s3.py
index be2d7d59..ddca269a 100644
--- a/tests/test_services_backup_s3.py
+++ b/tests/test_services_backup_s3.py
@@ -78,6 +78,22 @@ def test_a_refusal_says_why(tmp_path):
         asyncio.run(s3.put_file(TARGET, "b.tar", f, transport=transport))
 
 
+def test_a_web_server_at_the_address_is_named_as_not_s3():
+    """No port in the address: the request lands on whatever serves port 80,
+    and its plain 404 says nothing about S3."""
+    page = b"<html><body><h1>404 Not Found</h1></body></html>"
+    transport = httpx.MockTransport(lambda r: httpx.Response(404, content=page))
+    with pytest.raises(s3.S3Error, match=r"HTTP 404: the address answered, but not as S3 — check the scheme and the port"):
+        asyncio.run(s3.list_objects(TARGET, transport=transport))
+
+
+def test_a_server_error_without_a_body_is_not_blamed_on_the_port():
+    transport = httpx.MockTransport(lambda r: httpx.Response(503))
+    with pytest.raises(s3.S3Error, match="HTTP 503: Service Unavailable") as err:
+        asyncio.run(s3.list_objects(TARGET, transport=transport))
+    assert "port" not in str(err.value)
+
+
 def test_unreachable_says_so(tmp_path):
     f = tmp_path / "b.tar"
     f.write_bytes(b"x")