Hi @al64.
The corrupted line from the Serial Monitor should have said:
Brownout detector was triggered
If this is appearing while the ESP32 board is connected to your computer via the USB isolator, but does not appear while the board is directly connected, it indicates that the current the isolator is capable of supplying is less than what is required by your system.
This is not surprising as the isolator is only rated for 100 mA and even the ESP32 alone can momentarily draw more current than that when using the Wi-Fi radio.
The "brownout detector" is a system in the ESP32 microcontroller that automatically resets it if it detects that the operating voltage has dropped below the minimum level required for reliable operation. This is done because if the voltage drops below the minimum level (but not to 0 V), then the microcontroller can be left in a non-functional state even after the voltage increases to a level above the minimum. In this case, it is better to just reset to restore functionality.