Unpredictable Pauses during Report Processing (BC14)

Hi one of our customer complains about delayed output of RDLC-reports printed on local or network printers. This happens at unpredictable times, but several times a day (per user). Often a customer of our customer is waiting to get the paper report (shipment or invoice). The time varies from some seconds up to 10 minutes.
In some cases we saw that another user starts processing a different customer and finishes the process and gets his printout in a few seconds (on the same printer) prior the one who already waits some minutes. The first started printout arrives minutes later...

First we enhanced the C/AL code of some reports to collect log information (which report was printed at what time, which trigger was executed and some report data information. We put this log-code on some OnPre and OnPost triggers. The logs showed that the delay occured always either between “OnPreReport” and the first “OnPreDataItem” or / and between the last “OnPostDataItem” and “OnPostReport”.
We were able to connect the debugger to some waiting processes. The debugger sits and waits for minutes - not showing anything. If it came to live its always the beginning of the first “OnPreDataItem”- or the “OnPostReport”-trigger.

Has anyone experienced such a behaviour and/or can give some advice how to further hunt the issue or resolve it?
The customer is on BC 14.0.44327

Thanks in advance
Heinrich