Hi, thanks for the reply!
I actually updated from 4.10.6-24 (to 5.0.0-95)

I have discovered this via JUnit testing (I did not deploy to the production, and will probably not as the risk is relatively high).
The test suite is starting a server and creating a client once when initialized:

private static UaServer server;
private static UaClient client;

@BeforeAll
static void setup() throws Exception {
// create server
server = new UaServer();
server.setPort(14840);
server.setServerName(“OPCUA/Test”);
server.getSecurityModes().add(SecurityMode.NONE);
server.setUserTokenPolicies(UserTokenPolicies.SECURE_USERNAME_PASSWORD);
var appDescription = new ApplicationDescription();
appDescription.setApplicationName(new LocalizedText(“TestServer”));
appDescription.setApplicationUri(“urn:test:opcua”);
appDescription.setProductUri(“https://test.com/opcua”);
appDescription.setApplicationType(ApplicationType.Server);
server.setApplicationIdentity(ApplicationIdentity.createCertificate(appDescription, “Test”, 512));

server.start();

// create client
client = new UaClient(“opc.tcp://127.0.0.1:14840/OPCUA/Test”);
client.setApplicationIdentity(new ApplicationIdentity());
client.setSecurityMode(SecurityMode.NONE);
client.setUserIdentity(new UserIdentity(USERNAME, PASSWORD));
client.setTimeout(10, TimeUnit.MINUTES);
}

For each test I create a new connection:

@BeforeEach
void connect() throws ServiceException {
client.connect();
}

And I disconnect it after each test:

@AfterEach
void disconnect() {
if (client.isConnected()) {
client.disconnect();
}
}

(I tried to add Thread.sleep(1000) here to give the disconnecting some time, but it did not help)

Then, there are about 10 tests that connect to the server and read/write some data or attributes:

@Test
void browses_deviceSet_node() throws Exception {
var references = client.getAddressSpace().browse(new NodeId(nsIndex, “DeviceSet”));
assertThat(references).hasSize(2);
var displayNames = references.stream()
.map(ReferenceDescription::getDisplayName)
.map(LocalizedText::getText)
.collect(Collectors.toSet());
assertThat(displayNames).contains(“Device1”, “Device2”);
}

@Test
void reads_device_attributes() throws Exception {
var browseName = client.readAttribute(new NodeId(nsIndex, DEVICE_NODE_ID_REAL_1), Attributes.BrowseName);
assertAll(
() -> assertThat(browseName.getStatusCode().getValue().getValue()).isEqualTo(StatusCodes.Good.getValue()),
() -> assertThat(((QualifiedName) browseName.getValue().getValue()).getName()).isEqualTo(“Device1”)
);
}

@Test
void writes_serialNumber_value() throws Exception {
var value = client.writeValue(new NodeId(nsIndex, “Device1/SerialNumber”), “Updated”);
assertAll(
() -> assertThat(value).isTrue(),
() -> assertThat(currentValue).isEqualTo(“Updated”)
);
}

Not really very demanding, but one or two tests fail with the mentioned exception almost every run of the test suite.

And it seems that client.setStatusCheckInterval(0); really “fixed” the issue. I do not get the error anymore, all is working fine as before the update.
Of course, this must be fixed in the server SDK…

Thanks. I will then wait for the bug release.
If you can later put here some details when the problem is found, it would be interesting for me!