Hi,

The log message means that the delete command received refers to a key that is not present in the internal model of the Item maintained by the Lightstreamer server.
This means that either the key has never been added or has already been deleted before.
You should double check that the key is exactly identical to the one provided in a previous ADD event.
Please also consider that an UPDATE event for a missing key is automatically converted into an ADD event.

But if not the case, you should reproduce the issue in a test environment, with a limited number of active clients, and activate this log configuration:

<logger name="LightstreamerLogger.subscriptions" level="DEBUG"/>
<logger name="LightstreamerLogger.subscriptions.upd" level="DEBUG"/>

then please provide us the Lightstremaer server log.

Regards,
Giuseppe