Program does not start correctly with GPIO4 on high.

[olimex]

Program does not start correctly with GPIO4 on high.

[DanKoloff]

Look at the "Internal Bootstrapping Resistors" table GPIO4 is present and has a Pull-Down. Maybe this bugs the converter and the logic.

You can use one of the other pins at the UEXT connector instead of GPIO4 - Like all peripherals, the pins for the UARTs can be logically mapped to any of the available pins on the ESP32.

For example, try using GPIO16 (pin #6 from the UEXT); of course remember to change it in the code.

[nagyrobi]

For reference, from: https://www.olimex.com/forum/index.php?topic=8093.0
It seems like adding 1K pull down resistor between GPIO4 and GND fixes the issue.
This has helped me connecting a MODBUS-TTL adapter to ESP32-EVB UEXT connector's GPIO4.