Thanks, @kuzulis for giving me a reference to look at the device side. The problem is solved by adding line coding settings to the CDC buffer on the STM. It is related to the hardware. STM users should bind USBD_CDC_LineCodingTypeDef data structure to CDC_Control_FS function. In ready-made examples, this binding does not exist. Users who do not add this binding can face the same problem in long term tests. Maybe it can help people who face the same problem. Regards