diff --git a/app/services/workflow_service.py b/app/services/workflow_service.py
index a460329..dabd8ad 100644
--- a/app/services/workflow_service.py
+++ b/app/services/workflow_service.py
@@ -401,14 +401,25 @@ async def list_instances(
     page: int = 1,
     page_size: int = 20,
     status_filter: str | None = None,
+    user_id: uuid.UUID | None = None,
+    is_system_admin: bool = False,
 ) -> dict[str, Any]:
-    """List workflow instances with optional status filter."""
+    """List workflow instances with optional status filter.
+
+    BUG-036 fix: the route passed ``user_id``/``is_system_admin`` which this
+    signature did not accept -> TypeError -> 500 on every call.
+
+    Non-admin users see only instances they initiated; admins/system admins
+    see all instances of the tenant.
+    """
     page = max(1, page)
     page_size = max(1, min(100, page_size))
 
     base = select(WorkflowInstance).where(WorkflowInstance.tenant_id == tenant_id)
     if status_filter:
         base = base.where(WorkflowInstance.status == status_filter)
+    if not is_system_admin and user_id is not None:
+        base = base.where(WorkflowInstance.initiated_by == user_id)
 
     count_q = select(func.count()).select_from(base.subquery())
     total_result = await db.execute(count_q)
diff --git a/docs/test-bugs.md b/docs/test-bugs.md
index a77aa1c..1d1b374 100644
--- a/docs/test-bugs.md
+++ b/docs/test-bugs.md
@@ -353,7 +353,7 @@ Jeder Bug wird wie folgt dokumentiert:
 - **Response:** `{"code":"internal_error","detail":"Internal server error","trace_id":"bbdf0698"}`
 - **Schweregrad:** High
 - **Ursache:** Unbekannt — muss Backend-Log prüfen
-- **Status:** ⏳ Nicht gefixt
+- **Status:** ✅ Gefixt 2026-08-24 (Block E/I-C): Root-Cause bewiesen — Route übergab user_id/is_system_admin an list_instances, Service-Signatur akzeptierte beide nicht → TypeError → 500 auf JEDEN Aufruf. Fix: Service-Signatur erweitert um optionale user_id/is_system_admin; Nicht-Admins sehen nur eigene Instanzen (initiated_by-Filter), Admins alle; agent_runner-Caller bleibt kompatibel. Beweistest test_bug036_instances.py 2/2 grün (200 plain + 200 mit status-filter); ruff clean.
 
 ### BUG-037: Compliance Incident POST gibt 500 Internal Server Error
 - **Kategorie:** API
diff --git a/tests/test_bug036_instances.py b/tests/test_bug036_instances.py
new file mode 100644
index 0000000..1ad0489
--- /dev/null
+++ b/tests/test_bug036_instances.py
@@ -0,0 +1,35 @@
+"""Tests for BUG-036 fix — GET /api/v1/workflows/instances returned 500.
+
+Root cause: the route passed ``user_id``/``is_system_admin`` which the service
+signature did not accept -> TypeError -> 500 on every call.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from tests.conftest import ORIGIN_HEADER, login_client, seed_tenant_and_users
+
+
+@pytest.mark.asyncio
+async def test_list_instances_returns_200(client, db_session):
+    """GET /api/v1/workflows/instances must return 200 (was 500 before fix)."""
+    await seed_tenant_and_users(db_session)
+    await login_client(client, "admin@tenanta.com")
+
+    resp = await client.get("/api/v1/workflows/instances", headers=ORIGIN_HEADER)
+    assert resp.status_code == 200, resp.text
+    data = resp.json()
+    assert "items" in data and "total" in data
+
+
+@pytest.mark.asyncio
+async def test_list_instances_status_filter_returns_200(client, db_session):
+    """Status filter variant also works after the signature fix."""
+    await seed_tenant_and_users(db_session)
+    await login_client(client, "admin@tenanta.com")
+
+    resp = await client.get(
+        "/api/v1/workflows/instances?status=running", headers=ORIGIN_HEADER
+    )
+    assert resp.status_code == 200, resp.text