RS485 communication issue
Based on the description, there are only the differential data wires A and B connected between the two RS485 tranceivers that do not share a common reference, as one of the tranceivers includes isolation and runs on separate isolated power supply, so it completely floats in regards to the other tranceiver and the voltages float more than the common mode range allows for.
RS485 tranceivers must share a common voltage reference, and this is why there should be a common wire between tranceiver, that connects the ground of isolated RS485 tranceiver to ground of the other tranceiver, so they have a common reference potential.