Until some minutes ago, everything worked perfectly fine, but suddenly the M5Stack (alone, as well with modules attached), stopped working while connected to a USB port of my computer and/or charger.

When I connect a battery and disconnect the USB cable, it boots up. As soon as I reconnect the USB cable, it switches off... Remove the USB cable: It boots again...

The USB serial port is still visible when connected to the computer, but there is no response from the ESP. To me this looks as if it is really without power supply. Is there some comparator IC, which selects the power source, which might be confused?

Does anyone have any ideas what the problem could be and how it could be fixed?

Change the pins for the GPS module as follows (by cutting the existing bridge and re-soldering). Only downside is, that the Serial monitor/output is not available anymore

• RXD: 1
• TXD: 3
• PPS (not connected)

Now, you can use the GPS module on the serial port 0:

// The TinyGPS++ object
TinyGPSPlus gps;
// The serial connection to the GPS device
HardwareSerial gps_serial(0);
gps_serial(GPSBaud);

// ... (within a loop/smart delay)
while (gps_serial.available())
    gps.encode(gps_serial.read());

For the SIM800L you can use the regular configuration without changing anything:

...
#define SerialAT Serial2
TinyGsm modem(SerialAT);
TinyGsmClient client(modem);
...