The interval that defines the fastest rate at which the MonitoredItem(s) should be accessed and evaluated. This interval is defined in milliseconds.
The value 0 indicates that the Server should use the fastest practical rate.
The value -1 indicates that the default sampling interval defined by the publishing interval of the Subscription is requested. A different sampling interval is used if the publishing interval is not a supported sampling interval. Any negative number is interpreted as -1. The sampling interval is not changed if the publishing interval is changed by a subsequent call to the ModifySubscription Service.
The Server uses this parameter to assign the MonitoredItems to a sampling interval that it supports.
The assigned interval is provided in the revisedSamplingInterval parameter. The Server shall always return a revisedSamplingInterval that is equal or higher than the requested samplingInterval. If the requested samplingInterval is higher than the maximum sampling interval supported by the Server, the maximum sampling interval is returned.
@jeromevalentin There could be several causes here:
Usually this warning can be ignored, but it show some behavior that may hide a wider problem so it is good to understand what is causing it.