Many thanks to all those who responded, first of all. Based on the initial comment by @wally, I made some further tests yesterday night. Indeed, the car kit has a button/function to import the contacts from the phone, but it fails to import anything. Its message is that maybe I need to authorize the sharing on the phone. I also tried to export all contacts to vCard on the phone (successfully), then to browse to that file in file manager, then shift-left and click on the sharing icon, but that only listed "Messages" and "Dekko" as potential targets. New import attempts from the car kit ended up with the same error message. So I guess that this "sharing" icon is where a "BT File Transfer" app would need to register itself. For the record, this is how I used to send contact info to the car kit from my Nokia N900, using a BT transfer app called "Petrovich'.

@PhAndersson Congrats and welcome!

@Moem Thanks a lot for your suggestion -- I'll have a look at it.

When a Xiaomi Mi A2 I used did not charge anymore or only very slowly, the problem was fixed by using a different USB cable and a stronger charger. The old USB cable and charger still worked perfectly fine with other mobile phones, so this was clearly an issue with the Xiaomi Mi A2. This was under Android, so I think this is a hardware problem, that can appear after some time.

Status update: As it happens, I took the opportunity of having the phone on the dev channel to re-test an application that typically froze the GUI (the app called "Steps" -- I already opened a bug report on that specific issue). Turns out the behaviour was the same on v763, so I wanted to force a power cycle by rebooting the phone on the recovery image (Power + Vol+). But instead of the recovery menu, I was greeted by the spiralling animation: the phone was clearly installing something. When it finished rebooting, I was back on OTA-20, and both SIMs were still active! Following @DPITTI suggestion, I also removed v763 from the upgrade history. I'll now mark this post as "resolved". Many thanks to all those who contributed.

@phandersson said in UBports install fails with permission error on the device: or would UT also rely on those same drivers to access the HW? I had the same problem and to solve it, I had to format partition in TWRP and then restart UPI.

Problem has been resolved -- phone is now running UT OTA-19 stable. Details about the fix in the device-specific post (see URL in previous post). I'll now mark this one as resolved as well. Many thanks to all those who helped.

@phandersson Hurray! Good job, but very strange...

@phandersson Just some stuff you have to do to unlock the bootloader of an Xiaomi phone I have discovered: the phone must have a working SIM inserted You must have a Mi Account You must link the phone to that Mi account in order to unlock the bootloader As per usual, developer options must be enabled, USB debugging enabled and also OEM unlock enabled You can then unlock the bootloader, follow the instructions on the phone to the letter, and also follow the Ubuntu Touch installer instructions. You have to download the Mi unlock tool and run that with the phone connected (obviously).