Answer by Mauro for Chrome doesn't open automatically magnet links anymore

Ok, I solved this: instead of changing "protocol_handler":{} to "protocol_handler":{"excluded_schemes":{"magnet":false}}, I left it unchanged and added"protocol_handler":{"excluded_schemes":{"magnet":false}} (having then two "protocol_handlers").

Edit: For this to work, you have to add "protocol_handler":{"excluded_schemes":{"magnet":false}}, (with the final comma) after the initial { in C:\Users\Mauro\AppData\Local\Google\Chrome\User Data\[user]\Preferences. In other words, it needs to be at the root, and before the other "protocol_handler":{}.