Solved!
I’m very grateful to both of you for your help. 
The “Advanced” menu was not obvious to find in the recovery because, when the error message (of the recovery) is displayed, it only gives the options “Try again” or “Factory data reset”.
BUT there is an arrow on top left on the screen. That’s what I was missing: If you tap on it, then you have the advanced menu, where you can enable adb.
Then I used the command-lines in thread How to mount encrypted /data partition in recovery mode? to find the biggest files. I guessed they were the Signal backup files (and I was right). So I just removed 2 of them, and the phone could boot properly.
Such a relief!