Just joined to say the same
Arduino Leonardo / Micro Pro is not uploading
I don’t know why exactly, but on SOME pcs it just waits far too long (until the Arduino goes out of the bootloader) and only THEN tries to upload it, causing it to avrdude: ser_open(): can’t open device “.\COM5”.
A workaround is to manually reset the Arduino multiple times when it’s looking for the “New upload port” and if you do it in the right time, it recognizes it and uploads the firmware.
It’s annoying and only happens on some PCs.