Thank you for contacting the Toradex community!
Auto-detect baud rate is not supported by Windows API hence it’s not exposed from the driver.
Could you try to increase priority256 of the UART that need to support high-speed, you need to create the priority256 under the serial port registry section and assign lower value e.g. https://developer.toradex.com/knowledge-base/serial-port-registry-settings#Colibri_VFxx_Registry_Settings
Also, try to use different UART and let us know if you see the difference in the issue.
I.MX6 supports up to 5Mbit baud rate but our driver limited with 115200. If you want to try 5Mbits, you need to manually configure these two registers UBIR = 0x0003 and UBMR = 0x0004, after serial port API open calls. For more details, please refer : 64.13.2 Programming the UART in 9-bit RS-485 mode in IMX6 TRM.
This can be done using RegAccessTool or MapMem library.
If you increase the priority that would worsen the issue but if you decrease the baud rate that may enough CPU time to read from the FIFO and may suppress the overrun error.