NPrinting 17 and 18 reports are not sent - "Filters for request xxx produce an invalid selection" error in the logs

An NPrinting task contains a report that has to be sent to recipients. There are filters associated to the report, the recipients or to the task.
Some recipient do not receive the report.
The log files show a "Filters for request xxx produce an invalid selection" error in the logs.

Other errors that may be seen in the Engine logs when running the report with invalid selections follows:
...
ERROR: Error processing request of type Qlik.Reporting.Engine.Messages.Requests.FilterMaterializationRequest for sense app <app id>. ERROR: Cannot apply filter Filters:...
Bookmark: , Void: False to current document data. Requested fields with evaluates are: F\<field name>
...
Error processing request of type Qlik.Reporting.Engine.Messages.Requests.FormulaNodeRequest for sense app <app id>. ERROR: System.TimeoutException: Method "EvaluateEx" timed out
...

Environment:

• NPrinting 17 / 18

NPrinting does not generate a reports when the filters applied to it are incompatible. With incompatible we mean that the filter are not selecting any data. There are some typical cases in which we have incompatible filters:

• You are applying the same filter to different locations (for example to a user and to the task). Like double selections in QlikView, double filters would show all data.-
• You are applying filters with no data in common. A typical example is that you have a filter selecting Country= "Germany" and one selecting Country= "Italy". Generally, if you apply two different filters based on the same filed, the report is not generated
• You apply a selection on a value that does not exist or not exists anymore. This happens when the value you set in the filter is not present in the field. It may also  happen that a filter you created is not consistent after a change in the source document.  You may have created a Country= "Germany" field that used to work correctly, but a reload removed Germany form the Country field. After the reload, the filter will be incompatible.
• A filter may be incompatible with other filters you applied. This happens if the filter can work alone, but the selections applied before remove the values selected by the filter. A typical example: You have a filter Country= "Germany" to select Sales over the year in Germany and that works OK. Imagine that there are no sales for the month of April. If you add a Month="April" filter together with the Germany filter, you get incompatible selections.

NPrinting automatically checks for incompatible filters at the beginning of a task execution and automatically stops the report generation if there is any incompatibility. This is an optimization common to all NPrinting 17 and 18 releases.

When you use this kind of selections (incompatible filters), QlikView and Qlik Sense would remove all the selections and, if NPrinting would not apply this optimization, a report with all the data would be sent. This is not what customers want to do. It could also be a dangerous behavior because confidential data could be sent to people that are not supposed to see it.