If on Windows, this can happen if two apps try to open the same COM port with FILE_FLAG_OVERLAPPED not set properly. Close other serial monitoring tools.
This report details the current status of device drivers for the USB-to-Serial converter. u232 p9 driver exclusive
Finding the "exclusive" or correct driver can be difficult because these adapters often use different chipsets (MCT or Prolific) depending on the brand. Vadim Tsozik - MCT U232-P9 Linux driver If on Windows, this can happen if two
The is a foundational piece of hardware used to connect modern computers to legacy RS-232 serial devices. Finding, installing, and managing the correct u232 p9 driver exclusive software is critical to preventing communication failures, data corruption, or system crashes. If on Windows