When a cluster is restored from a snapshot, resourceClusterUpdate is called, passing the engine-version along to ModifyDBCluster. This will always fail, even when the engine-version has not changed. The solution might be to omit the version from the arguments in case the resource has been freshly created.

Author: danielcweber

internal/service/neptune/cluster.go

@@ -638,7 +638,7 @@ func resourceClusterUpdate(d *schema.ResourceData, meta interface{}) error {
 		requestUpdate = true
 	}
 
-	if d.HasChange("engine_version") {
+	if !d.IsNewResource() && d.HasChange("engine_version") {
 		req.EngineVersion = aws.String(d.Get("engine_version").(string))
 		requestUpdate = true
 	}