FP2 not rebooting after updating to 21 / Android 9

Thank you very much for your answer! I didn’t know there was no secure startup on FP2.
I was referring to these guidelines, where they do say that you have to disable secure startup first: Manually install Fairphone OS for the Fairphone 2 – Support
But it’s great if it’s not a problem.
In the mean time, I have tried a factory reset because I was beginning to feel hopeless (I have set an automatic backup via Google I think). But it didn’t worked.
I am now in the same situation as described here: Fairphone OS 19.11.2 update. Upgrade error: Unable to find partition for path '/system' - #5 by Aitziber
The similarity with this post is that I cannot find my phone on my computer via USB… I only have old USB cables with me at the moment and they don’t seem to work. Sometimes the phone appears in the device manager, but only briefly. And in ADB it doesn’t appear at all.
I don’t know what to do now. Do you have any idea?
I think there aren’t as many fairphone users in my native language (French) as in English, and I would not have had such a fast answer, so I wil make an effort with my English.
But maybe I should open a new topic as you suggested? I wasn’t sure if it was relevant since my issue seems similar to others