diff --git a/src/backend/admin_ops/__init__.py b/src/backend/admin_ops/__init__.py
index cd88dff5..45adbeff 100644
--- a/src/backend/admin_ops/__init__.py
+++ b/src/backend/admin_ops/__init__.py
@@ -18,6 +18,8 @@
 from botocore.auth import HmacV1Auth
 from botocore.awsrequest import AWSRequest
 from botocore.credentials import Credentials
+from fastapi import HTTPException, status
+from fastapi.logger import logger
 
 from backend.admin_ops.errors import error_from_response
 
@@ -63,10 +65,22 @@ def signed_request(
 
 async def send_request(req: httpx.Request) -> httpx.Response:
     async with httpx.AsyncClient(verify=False) as client:
-        res: httpx.Response = await client.send(req)
-        if not res.is_success:
-            raise error_from_response(res)
-        return res
+        try:
+            res: httpx.Response = await client.send(req)
+            if not res.is_success:
+                raise error_from_response(res)
+            return res
+        except httpx.ConnectError as e:
+            raise HTTPException(
+                status_code=status.HTTP_502_BAD_GATEWAY,
+                detail=str(e),
+            )
+        except Exception as e:
+            detail: str | None = str(e) or None
+            logger.error(f"Unknown exception ({type(e)}): {detail}")
+            raise HTTPException(
+                status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail=detail
+            )
 
 
 async def do_request(
diff --git a/src/backend/api/__init__.py b/src/backend/api/__init__.py
index a5720f64..e9d23c98 100644
--- a/src/backend/api/__init__.py
+++ b/src/backend/api/__init__.py
@@ -133,10 +133,11 @@ async def conn(self, attempts: int = 1) -> AsyncGenerator[S3Client, None]:
                 # propagate it.
                 raise e
             except Exception as e:
-                logger.error(f"Unknown error: {e}")
-                logger.error(f"  exception: {type(e)}")
+                detail: str | None = str(e) or None
+                logger.error(f"Unknown exception ({type(e)}): {detail}")
                 raise HTTPException(
-                    status_code=status.HTTP_500_INTERNAL_SERVER_ERROR
+                    status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+                    detail=detail,
                 )
 
 
diff --git a/src/backend/api/auth.py b/src/backend/api/auth.py
index a214689b..8333206d 100644
--- a/src/backend/api/auth.py
+++ b/src/backend/api/auth.py
@@ -14,7 +14,7 @@
 
 from typing import Annotated
 
-from fastapi import Depends, HTTPException, status
+from fastapi import Depends, HTTPException
 from fastapi.routing import APIRouter
 from types_aiobotocore_s3.type_defs import ListBucketsOutputTypeDef
 
@@ -36,8 +36,8 @@
 async def authenticate(conn: S3GWClientDep) -> AuthUser:
     # Try to access a RGW Admin Ops endpoint first. If that works, the
     # given credentials have `admin` privileges. If it fails, try to
-    # access a RGW endpoint. If that works, the given credentials can
-    # be used to sign in as `regular` user.
+    # access a RGW endpoint via AWS S3 API. If that works, the given
+    # credentials can be used to sign in as `regular` user.
     try:
         admin_ops_res: admin_ops_types.UserInfo = (
             await admin_ops_users.get_user_info(
@@ -53,14 +53,16 @@ async def authenticate(conn: S3GWClientDep) -> AuthUser:
             IsAdmin=admin_ops_res.admin,
         )
     except HTTPException:
-        try:
-            async with conn.conn() as s3:
-                s3_res: ListBucketsOutputTypeDef = await s3.list_buckets()
-                res = AuthUser(
-                    ID=s3_res["Owner"]["ID"],
-                    DisplayName=s3_res["Owner"]["DisplayName"],
-                    IsAdmin=False,
-                )
-        except HTTPException:
-            raise HTTPException(status_code=status.HTTP_403_FORBIDDEN)
+        # Handle ALL HTTP related exceptions here, e.g. connection errors.
+        # The `httpx` library used for calling the Admin Ops API does not
+        # provide meaningful error messages, whereas `botocore` provides
+        # connection error messages that might be more helpful when they
+        # are displayed in the UI.
+        async with conn.conn() as s3:
+            s3_res: ListBucketsOutputTypeDef = await s3.list_buckets()
+            res = AuthUser(
+                ID=s3_res["Owner"]["ID"],
+                DisplayName=s3_res["Owner"]["DisplayName"],
+                IsAdmin=False,
+            )
     return res
diff --git a/src/backend/tests/unit/admin_ops/test_buckets.py b/src/backend/tests/unit/admin_ops/test_buckets.py
index da7836a0..2744ffc6 100644
--- a/src/backend/tests/unit/admin_ops/test_buckets.py
+++ b/src/backend/tests/unit/admin_ops/test_buckets.py
@@ -16,7 +16,7 @@
 
 import httpx
 import pytest
-from fastapi import HTTPException
+from fastapi import HTTPException, status
 from pytest_httpx import HTTPXMock
 
 import backend.admin_ops.buckets as buckets
@@ -105,7 +105,7 @@ async def test_bucket_list(httpx_mock: HTTPXMock) -> None:
 @pytest.mark.anyio
 async def test_bucket_list_failure(httpx_mock: HTTPXMock) -> None:
     httpx_mock.add_response(  # pyright: ignore [reportUnknownMemberType]
-        status_code=404  # any error, really
+        status_code=status.HTTP_404_NOT_FOUND  # any error, really
     )
 
     with pytest.raises(HTTPException) as e:
@@ -115,7 +115,7 @@ async def test_bucket_list_failure(httpx_mock: HTTPXMock) -> None:
             secret_key="qwe",
             uid=None,
         )
-    assert e.value.status_code == 404
+    assert e.value.status_code == status.HTTP_500_INTERNAL_SERVER_ERROR
 
 
 @pytest.mark.anyio
@@ -126,7 +126,9 @@ def check_uid(req: httpx.Request) -> httpx.Response:
         nonlocal called_cb
         called_cb = True
         assert str(req.url.query).find("uid=asdasd") >= 0
-        return httpx.Response(status_code=200, json=bucket_list_response)
+        return httpx.Response(
+            status_code=status.HTTP_200_OK, json=bucket_list_response
+        )
 
     httpx_mock.add_callback(  # pyright: ignore [reportUnknownMemberType]
         check_uid
diff --git a/src/backend/tests/unit/admin_ops/test_users.py b/src/backend/tests/unit/admin_ops/test_users.py
index eb7fd48a..01fb553f 100644
--- a/src/backend/tests/unit/admin_ops/test_users.py
+++ b/src/backend/tests/unit/admin_ops/test_users.py
@@ -18,7 +18,7 @@
 
 import httpx
 import pytest
-from fastapi import HTTPException
+from fastapi import HTTPException, status
 from pytest_httpx import HTTPXMock
 from pytest_mock import MockerFixture
 
@@ -128,14 +128,14 @@ async def test_get_user_info_3(httpx_mock: HTTPXMock) -> None:
 @pytest.mark.anyio
 async def test_get_user_info_failure(httpx_mock: HTTPXMock) -> None:
     httpx_mock.add_response(  # pyright: ignore [reportUnknownMemberType]
-        status_code=404,  # any error, really
+        status_code=status.HTTP_404_NOT_FOUND,  # any error, really
     )
 
     with pytest.raises(HTTPException) as e:
         await get_user_info(
             url="http://foo.bar:123", access_key="asd", secret_key="qwe"
         )
-    assert e.value.status_code == 404
+    assert e.value.status_code == status.HTTP_500_INTERNAL_SERVER_ERROR
 
 
 @pytest.mark.anyio
diff --git a/src/backend/tests/unit/api/test_api_auth.py b/src/backend/tests/unit/api/test_api_auth.py
index c9e1b190..ab84db9f 100644
--- a/src/backend/tests/unit/api/test_api_auth.py
+++ b/src/backend/tests/unit/api/test_api_auth.py
@@ -13,7 +13,9 @@
 # limitations under the License.
 
 import pytest
-from fastapi import HTTPException
+from botocore.exceptions import EndpointConnectionError
+from fastapi import HTTPException, status
+from httpx import ConnectError
 from pytest_httpx import HTTPXMock
 from pytest_mock import MockerFixture
 
@@ -105,4 +107,25 @@ async def test_authenticate_3(
 
     with pytest.raises(HTTPException) as e:
         await auth.authenticate(s3_client)
-    assert e.value.status_code == 403
+    assert e.value.status_code == status.HTTP_500_INTERNAL_SERVER_ERROR
+
+
+@pytest.mark.anyio
+async def test_authenticate_4(
+    s3_client: S3GWClient, mocker: MockerFixture, httpx_mock: HTTPXMock
+) -> None:
+    httpx_mock.add_exception(
+        exception=ConnectError(message="All connection attempts failed")
+    )
+
+    s3api_mock = S3ApiMock(s3_client, mocker)
+    s3api_mock.patch(
+        "list_buckets",
+        side_effect=EndpointConnectionError(
+            endpoint_url="http://127.0.0.1:7481/"
+        ),
+    )
+
+    with pytest.raises(HTTPException) as e:
+        await auth.authenticate(s3_client)
+    assert e.value.status_code == status.HTTP_502_BAD_GATEWAY