Sketch won't upload to ESP32 (failed to connect)

[mw1234]

Hello everyone,

I have a SparkFun ESP32 Thing Board, with which I want to control some Neopixel LED. I'm new to the whole ESP32/Arduino topic.
For uploading the sketch from Arduino IDE to the ESP32, I have to hold the physical "boot button 0" on the board, until the sketch starts uploading:

esptool.py v.2.3.1 Connecting..... -> pushing boot button -> transfer starts

Everything worked fine until yesterday. Now, the sketch won't upload, even if I hold down the "boot button".
(-> A fatal error occurred: Failed to connect to ESP32: Timed out waiting for packet header).

Also the physical reset button doesn't seem to work anymore. (before: connection to WiFi gets refreshed by pushing the reset button)

The blue LED on the ESP32 at PIN 5 was blinking before, now it constantly glows when connecting to power.

What can I do to upload sketches to the ESP32?