@lakota Thank you for the advice. I crossposted this to the Telegram channel now.

Hi,

I used the ubports-installer tool to flash my MX4 from Ubuntu to UBports. I followed the onscreen instructions, enabled developer mode on the mx4, and initiated the process.

Right now, the ubports-installer tool shows a rotating circle around the mascot and on the MX4 there is a black screen with => FASTBOOT mode... written in the lower left corner.

However, the ubports-installer tool also has a gray bar below the mascot showing:
Meizu MX4 Please connect your device with an usb cable.
But wasn't that written there all the time? I did not pay attention on this.
Should I unplug the USB cable from my Mac or do I brick my phone by doing this?
How long will flashing approximately take?
Currently, I have waited about half an hour.

Thanks for your feedback.