diff --git a/backend/modules/tasks/operations/subtasks.js b/backend/modules/tasks/operations/subtasks.js
index b1c1bb8..54b8813 100644
--- a/backend/modules/tasks/operations/subtasks.js
+++ b/backend/modules/tasks/operations/subtasks.js
@@ -92,6 +92,26 @@ async function updateSubtasks(taskId, subtasks, userId) {
     );
 
     if (subtasksToDelete.length > 0) {
+        if (
+            subtasksToDelete.length === existingSubtasks.length &&
+            subtasks.length === 0
+        ) {
+            logError(
+                'WARNING: Attempting to delete all subtasks with empty array:',
+                {
+                    taskId,
+                    userId,
+                    existingCount: existingSubtasks.length,
+                    providedCount: subtasks.length,
+                    subtasksToDelete: subtasksToDelete.map((s) => ({
+                        id: s.id,
+                        name: s.name,
+                        parent_task_id: s.parent_task_id,
+                    })),
+                }
+            );
+        }
+
         await taskRepository.destroyMany({
             where: {
                 id: subtasksToDelete.map((s) => s.id),
diff --git a/backend/modules/tasks/utils/validation.js b/backend/modules/tasks/utils/validation.js
index ea6d7d3..73cd911 100644
--- a/backend/modules/tasks/utils/validation.js
+++ b/backend/modules/tasks/utils/validation.js
@@ -36,7 +36,18 @@ async function validateParentTaskAccess(parentTaskId, userId) {
         where: { id: parentTaskId, user_id: userId },
     });
     if (!parentTask) {
-        throw new Error('Invalid parent task.');
+        const anyTask = await Task.findOne({
+            where: { id: parentTaskId },
+        });
+        if (anyTask) {
+            throw new Error(
+                `Invalid parent task. Parent task exists but belongs to a different user (parent user_id: ${anyTask.user_id}, current user_id: ${userId}).`
+            );
+        } else {
+            throw new Error(
+                `Invalid parent task. Parent task with id ${parentTaskId} not found.`
+            );
+        }
     }
 
     const parentAccess = await permissionsService.getAccess(
@@ -49,7 +60,7 @@ async function validateParentTaskAccess(parentTaskId, userId) {
         isOwner || parentAccess === 'rw' || parentAccess === 'admin';
 
     if (!canWrite) {
-        throw new Error('Invalid parent task.');
+        throw new Error('Invalid parent task. Insufficient permissions.');
     }
 
     return parentTaskId;
diff --git a/frontend/components/Task/TaskDetails.tsx b/frontend/components/Task/TaskDetails.tsx
index ffd2c82..b849cc8 100644
--- a/frontend/components/Task/TaskDetails.tsx
+++ b/frontend/components/Task/TaskDetails.tsx
@@ -1062,7 +1062,10 @@ const TaskDetails: React.FC = () => {
                 );
                 if (existingIndex >= 0) {
                     const updatedTasks = [...tasksStore.tasks];
-                    updatedTasks[existingIndex] = updatedTask;
+                    updatedTasks[existingIndex] = {
+                        ...updatedTask,
+                        subtasks: updatedTask.subtasks || task.subtasks || [],
+                    };
                     tasksStore.setTasks(updatedTasks);
                 }
             }