rotemse - 2020-02-23

I have RPI 4, with the lastest Codesys version (3.5.15 patch 3)
I have Beckhoff slaves with ethercat running at 4ms cycle time.
the main program I set to 12ms cycle time.
Visu is 100ms.
I use USB/Serial with the CH340 driver.
I also downloaded the serial demo program from the Codesys website.

Everything seems to work, but at some point, I get no reply from the serial port. I send the data, but no reply.
I tried to close the serial port and reopen it, same problem. BUT when I reset the RPI (Cold reset from the Codesys development software) everything works again!

What could be the problem?