Wemos D1 Mini stalled: Failed to connect to ESP8266: Timed out waiting for packet header

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See if this thread solves your problem:

ESP 8266 timed out waiting for packet header

Solved the same issue, well it solves itself with a second attempt. Ensure serial monitor is closed. Reset board in flash mode. Hit upload and accept that it will fail the first time. Hit upload again and it works. I have experienced the same problem with ESP8266 01 and ESP8266 201, and for both, the second attempt to upload works every time.