KepserverEX - Janitza S2 modbus communication problem

We have a connection issue between KepserverEx 6.18 and Janitza UMG96-S2 power measuring device. The connection is working and the tags can be readed via QuickClient but during the reading the quality of some tags randomly switching from good to bad for a while. The strange is that not all of the tags quality switches and it happens randomly. Does anyone has a similar issue with a Modbus TCP connection?