FTDI not working while monitoring voltage with one pin

I'm powering Arduino Nano with external 5v. I have USB cable with disconnected power wire. If I power Arduino and plug my USB cable, everything works as expected. I'm using this Arduino to measure voltage with voltage divider. If one pin gets voltage (~2,8v) from divider and then I connect my USB cable I get "USB Device not recognized". If I connect that pin from voltage divider after connecting USB cable, everything works ok. How can I make FTDI work without first disconnecting pin from voltage divider?