In the future also you might get the certficate issue with Mirrorlink. Here is the solution: I knew what I discovered would be useful somehow. I came here just so you guys can try. I have a S8+ and a Peugeot 2008 17/17 with Mirrorlink connection and I was using all normally until few days ago. I started to get the message of Certificate Not Valid for this App. What I did? Go to SETTINGS>>APPLICATIONS and check the SHOW SYSTEM FILE. Search for Samsung Mirrorlink 1.1 and delete CACHE and DATA.
Once I connected to my car again I noticed the message wasn't being shown anymore. With the phone connected, I tap on the screen and saw there was a message "Mirrorlink needs to use Data to download a file" (or something aroudn those lines. I tap AGREE and the file was downloaded.
Removed the device and reconnected and VOIALA! Mirrorlink was working again. Also for everyone with Samsung and mirrorlink, try to use this process I video taped: