I did a little bit of research. Contacts communicates with ofono (the modem interface) via D-bus.
So, just started dbus-monitor and then tried to import the contacts. Contacts correctly sends the request (ofono documentation

method call time=1537922880.561822 sender=:1.106 -> destination=org.ofono serial=2 path=/ril_0; interface=org.ofono.Phonebook; member=Import

but there was no answer. When I tried to do the import for the second time, an answer came:

method call time=1537922880.561822 sender=:1.106 -> destination=org.ofono serial=2 path=/ril_0; interface=org.ofono.Phonebook; member=Import
error time=1537922880.561933 sender=:1.44 -> destination=:1.105 error_name=org.ofono.Error.InProgress reply_serial=2
string "Operation already in progress"
error time=1537922880.562014 sender=org.freedesktop.DBus -> destination=:1.44 error_name=org.freedesktop.DBus.Error.ServiceUnknown reply_serial=257
string "The name :1.105 was not provided by any .service files"

It was an error that the request is already in progress, however ofono for some reason sent it to a different destination. Therefore, D-bus didn't forward it to Contacts.
I also tried to wait longer (maybe just the import takes longer), but even after 5 hours the answer was "already in progress" (with the wrong destination, as well).
Then I found an old SIM in my drawer which was already blocked but it had some saved contacts. I tried the same procedure on it and the correct answer arrived almost instantly:

method call time=1537944623.758662 sender=:1.106 -> destination=org.ofono serial=2 path=/ril_0; interface=org.ofono.Phonebook; member=Import
method return time=1537944623.759495 sender=:1.43 -> destination=:1.106 serial=263 reply_serial=2
string "BEGIN:VCARD VERSION:3.0 <OUTPUT OMITTED>"

So, maybe my SIM is damaged after all. I will try to look into ofono why it gets stuck and why it sends the answer with wrong destination.

@unisuperbox said in I have ended my contract with the UBports Foundation:

I'm all too familiar with the speculation when a project leader resigns.

My first thoughts: he's either got a better job or he's been cancelled

From all the self-help books I've read I've learned 2 main lessons: always take responsibility for your life (i.e. don't blame others if things are not going your way) and self-love. Without responsibility you are basically giving up your freedom of choice (you become a victim) and without self-love you can't do what's best for you (e.g. would you want to harm your body by unhealthy food or behavior if you really love yourself?).
Following these rules helped me a lot and seems like you are now also acting according to them. I see a nice future ahead of you

Thank you for everything you've done for the community.

Managed to find a workaround. If you restart ofono (sudo initctl restart ofono), the import suddenly starts working. The drawback is that your phone goes offline and you will have to restart to get it back online.
The pattern seems to be, that if your SIM is offline, import works. My 2nd SIM is blocked, therefore it stood offline and the import worked on the first try.
Unfortunately, switching to flight mode doesn't work, because it simulates a modem with no SIM card.

Device: bq e5
Build: 2020-W49

Hello,
my device charges very slowly (after 10 hours of charging it was only on 85%). I needed to charge only once so far, but I've never experienced this before. I will try to switch back to the stable build if the next charging time doesn't improve.

@advocatux thanks, worked.
One comment for those who are doing the same thing: don't forget to enable developer mode once you have installed the legacy Ubuntu Touch. Otherwise UBports installer won't detect your device.