[Server] crash during subscription/session teardown in a multi-session telemetry scenario

[cristipogacean]

Type of issue

• Bug
• Enhancement
• Compliance
• Question
• Help wanted

Current Behavior

We are seeing a server-side crash in the OPC UA .NET Standard stack during subscription/session teardown in a multi-session telemetry scenario.

The first hard failure is:

System.NullReferenceException: Object reference not set to an instance of an object.
at Opc.Ua.Server.SubscriptionManager.PublishSubscriptionsAsync(Int32 sleepCycle, CancellationToken cancellationToken)

After that, the server starts returning repeated service faults and the client side begins reporting BadTooManyPublishRequests. In our end-to-end test this causes telemetry to stop flowing and the test eventually times out.

At the moment this looks like a stack issue in the server publish/subscription path rather than an application-level timeout.

Actual behavior

The publish task exits unexpectedly with NullReferenceException, then the server starts returning service faults and clients observe repeated BadTooManyPublishRequests. and seem to no longer recover until a restart.

Relevant log excerpts

<6>2026-03-12T05:44:34.689Z - Subscription ITEMS DELETED, Id=3193098814, ItemCount=0, ItemsToCheck=0, ItemsToPublish=0
<6>2026-03-12T05:44:34.691Z - Subscription DELETED, SessionId=ns=9;i=213501626, Id=3193098814, SeqNo=6, MessageCount=1
<6>2026-03-12T05:44:34.691Z - Subscription ITEMS DELETED, Id=3193098814, ItemCount=0, ItemsToCheck=0, ItemsToPublish=0
<6>2026-03-12T05:44:34.692Z - Subscription ITEMS DELETED, Id=3193098813, ItemCount=0, ItemsToCheck=0, ItemsToPublish=0
<6>2026-03-12T05:44:34.694Z - Subscription DELETED, SessionId=ns=9;i=213501626, Id=3193098813, SeqNo=6, MessageCount=1
<6>2026-03-12T05:44:34.694Z - Subscription ITEMS DELETED, Id=3193098813, ItemCount=0, ItemsToCheck=0, ItemsToPublish=0
<6>2026-03-12T05:44:34.705Z - Session CLOSED, Id=ns=9;i=213501626, Name=1234567890, ChannelId=f9da07b2-e2e0-4d62-aafb-a1043071c660-264, User=Anonymous
<6>2026-03-12T05:44:34.708Z - TCPSERVERCHANNEL ProcessCloseSecureChannelRequest success, ChannelId=264, TokenId=1, Socket=03F600C3
<6>2026-03-12T05:44:34.709Z - ChannelId 264: closed
<3>2026-03-12T05:44:34.705Z - Subscription - Publish Task (null) Exited Unexpectedly.
System.NullReferenceException: Object reference not set to an instance of an object.
   at Opc.Ua.Server.SubscriptionManager.PublishSubscriptionsAsync(Int32 sleepCycle, CancellationToken cancellationToken)

Expected Behavior

Deleting subscriptions / closing sessions should not crash the publish task.
The server should continue handling publish requests normally for remaining or newly created sessions.

Steps To Reproduce

No response

Environment

- Nuget Version: 1.5.378.106

Anything else?

No response