Little update on my problem :
I managed to copy the important data of the internal storage on my SD card via TWRP 
After backing up the data, I tried to factory reset the phone via TWRP (as suggested in this HOWTO, solution 3). It didn’t work, my phone is still in bootloop.
I tried all solutions listed in the HOWTO except the last one (as I’m not very confident with reinstalling the OS from the computer), and nothing worked.
I’m still waiting for an answer of the helpdesk…
What should I do ? I don’t have a functioning phone since 5 days, and it begins to be complicated for me 
I don’t understand what caused the problem, I only accepted the /e/ upgrade to Android 10 I was offered. And I don’t know what to do now…
I would be very grateful if someone could help me.
Thanks in advance and thanks to everyone who already helped me