That a difference exists in potential between the GND on the output of one power supply vs the other. That would mean that some parts in your system will experience a potential difference (voltage) that may far exceed their maximum ratings, and even if they don't burst into flames, there will be severe communications problems.
In this context the former. The GND of your 230V outlet is a safety measure and if your project is built into a metal enclosure, this needs to be connected with the safety GND. But it's a different topic.