I'm having same major issues with my UART. I have a ESP-01S module, connected to an Atmega328p via the UART; the Atmega TX line has a 2.2k/4.4k Ohm voltage divider to divide the voltage for the ESP. With the ESP in reset or with Serial non-initialized, the transmission works fine, as viewed by oscilloscope or a serial-to-USB cable. With the ESP actually reading, this is what I get: https://i.imgur.com/nboG7nz.jpg
-- any ideas? As expected, those not-quite-low bits read as ones, and so a noteworthy percentage of all bits are invalid.