@mndis You might be able to sidestep these errors by installing adb and fastboot:
sudo apt install android-tools-adb android-tools-fastboot
@ernest I checked the settings and found that the APN wasn't correctly propagated. After I fixed it, mobile data worked!
@advocatux Unless others continue to have this problem, I recommend that the issue be closed again.
By the way, Ernest's suggestion also led to fixing the problem with MMS messages (see issues 12, 276 and 976).
Thanks for everyone's help.
@Lakotaubp My earlier attempt was with RC, so I gave it a try with development. The good news: installation of development (2018-12-18/2) and then of stable worked flawlessly. The bad news: in neither version did mobile data work.
@advocatux I just added a comment to the issue, including the standard logs, but I didn't see any way to reopen the issue (I'm guessing I don't have the permissions). Would you please take care of that?
@Lakotaubp I tried following one of the suggestions in the thread, along the lines of your suggestion. I installed from the RC channel (W50) and then I attempted to reinstall from the stable channel. Unfortunately, the phone wound up in a bootloop and the only way I found to get it running again was to install factory Android (wiping the data) and then install UT again.