diff --git a/donkey/src/main/java/com/mirth/connect/donkey/server/channel/PollConnector.java b/donkey/src/main/java/com/mirth/connect/donkey/server/channel/PollConnector.java
index d279d274a0..984d80ef63 100644
--- a/donkey/src/main/java/com/mirth/connect/donkey/server/channel/PollConnector.java
+++ b/donkey/src/main/java/com/mirth/connect/donkey/server/channel/PollConnector.java
@@ -67,6 +67,11 @@ public void halt() throws ConnectorTaskException, InterruptedException {
         if (scheduler != null) {
             try {
                 scheduler.interrupt(job.getKey());
+                // Release the scheduler as stop() does, without waiting for the interrupted job.
+                // StdSchedulerFactory keeps a running scheduler under the channel ID, so the next
+                // deploy would get this one back with the poll job still stored and fail with
+                // ObjectAlreadyExistsException. See OpenIntegrationEngine/engine#571.
+                scheduler.shutdown(false);
             } catch (Exception e) {
                 throw new ConnectorTaskException(e);
             }
diff --git a/donkey/src/test/java/com/mirth/connect/donkey/server/channel/PollConnectorHaltTest.java b/donkey/src/test/java/com/mirth/connect/donkey/server/channel/PollConnectorHaltTest.java
new file mode 100644
index 0000000000..4d7e3855ca
--- /dev/null
+++ b/donkey/src/test/java/com/mirth/connect/donkey/server/channel/PollConnectorHaltTest.java
@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: MPL-2.0
+// SPDX-FileCopyrightText: Open Integration Engine Contributors
+
+package com.mirth.connect.donkey.server.channel;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import org.junit.Test;
+
+import com.mirth.connect.donkey.server.event.EventDispatcher;
+import com.mirth.connect.donkey.test.util.TestPollConnectorProperties;
+
+/**
+ * A halted poll connector must release its Quartz scheduler, so the channel can be deployed and
+ * started again on the same engine. See OpenIntegrationEngine/engine#571.
+ */
+public class PollConnectorHaltTest {
+
+    private static class NoopPollConnector extends PollConnector {
+        @Override
+        public void onDeploy() {}
+
+        @Override
+        public void onUndeploy() {}
+
+        @Override
+        public void onStart() {}
+
+        @Override
+        public void onStop() {}
+
+        @Override
+        public void onHalt() {}
+
+        @Override
+        protected void poll() {}
+
+        @Override
+        public void handleRecoveredResponse(DispatchResult dispatchResult) {}
+    }
+
+    private static PollConnector connector(String channelId) {
+        Channel channel = mock(Channel.class);
+        when(channel.getEventDispatcher()).thenReturn(mock(EventDispatcher.class));
+
+        PollConnector connector = new NoopPollConnector();
+        connector.setChannelId(channelId);
+        connector.setChannel(channel);
+        connector.setConnectorProperties(new TestPollConnectorProperties());
+        return connector;
+    }
+
+    /** Before the fix, the second start threw ObjectAlreadyExistsException for the poll job. */
+    @Test
+    public void startAfterHalt_onANewConnectorForTheSameChannel() throws Exception {
+        String channelId = "poll-halt-redeploy-test";
+
+        PollConnector first = connector(channelId);
+        first.start();
+        first.halt();
+
+        PollConnector second = connector(channelId);
+        second.start();
+        second.stop();
+    }
+
+    /** Halt then Start on the same connector, as the Dashboard does without undeploying. */
+    @Test
+    public void startAfterHalt_onTheSameConnector() throws Exception {
+        PollConnector connector = connector("poll-halt-restart-test");
+        connector.start();
+        connector.halt();
+
+        connector.start();
+        connector.stop();
+    }
+
+    /** Stop already released the scheduler; it keeps working. */
+    @Test
+    public void startAfterStop_onANewConnectorForTheSameChannel() throws Exception {
+        String channelId = "poll-stop-redeploy-test";
+
+        PollConnector first = connector(channelId);
+        first.start();
+        first.stop();
+
+        PollConnector second = connector(channelId);
+        second.start();
+        second.stop();
+    }
+}