How to decrypt an undecryptable micro SD card?
I finally find a solution in an recovery-android.com comment by vid971 :


  
  Go to Lock Screen and Security and select None and Keep Fingerprints. You will then be asked for setting up a password.
  Remove the SD Card from the phone and Reboot.
  When phone has rebooted put your SD Card back in. You will have windows prompts, for you Sim card Pin and Mobile Data usage, and after
  that you'll be on the Encrypt SD Card page. Click Encrypt. As your
  card is already encrypted it will be fast.
  
  
  From then your SD Card should be recognised again and usable.