Problem solved:
using the code in the start of main.py in this way:
import time
import struct
import array, random
from machine import Pin, UART
import rp2
import utime
import os
use_bme280 = True
use_mcp9808 = False
my_debug = False
# Micropython script for a Seeed XIAO RP2350 attached to a Seeed Expansion Board Base
# Test to receive ntp unixtime from another device: Pimoroni Pico Plus 2 with RM2 module attached
# also display/print sensor values from either mcp9808 or bme280 (see global flags below).
# by Paulus Schulinck (Github handle: @PaulskPt)
# 2025-05-08
# License: MIT
# Note: script uses modules in /sd/lib. The script in boot.py tries to mount the SDCard
#
# See: https://github.com/orgs/micropython/discussions/15292
# Post: "SOLUTION" by @wbeebe
# UPDATE 18 June 2024
"""
Notes PaulskPt:
After reading the "SOLUTION" by @wbeebe, I tried the following in Thonny Shell:
>>> import machine
>>> i0=machine.I2C(0)
>>> i0
I2C(0, freq=400000, scl=5, sda=4, timeout=50000)
>>>
>>> i1=machine.I2C(1)
>>> i1
I2C(1, freq=100000, scl=7, sda=6, timeout=50000)
>>>
So, instead of:
PIN_WIRE1_SCL = machine.Pin.board.GP7
PIN_WIRE1_SDA = machine.Pin.board.GP6
i2c = machine.I2C(id=1, scl=machine.Pin(PIN_WIRE1_SCL),
sda=machine.Pin(PIN_WIRE1_SDA), freq=100000)
I used:
i2c=machine.I2C(1)
That worked!
"""
i2c=machine.I2C(1)
# more code here...
Now main.py is starting after a power-on or a press of the reset button on the Seeed Expansion Board Base.
Case closed!