I get this error on my second car. The first car can send commands, but I cannot get the second one to work. Does the app I created in the tesla dev portal only work for one car?
The Tesla dev api key is used for the Homey. The Homey-ID is used to provide the certificate.
So on first car you can insert client_id and key.
On 2nd car, the client_id and key is already set and you can continue with your Tesla car account.
Both cars are using the app ID and are counted on your 10€ debit. That’s a little disadvantage.
To your issue:
It seems, the private key (certificate) could not be read. Did you somehow change the client_id or secret while adding the car?
Both cars are using the same key. So if one car is ok and the other not, then I have no idea what’s wrong.
Can you please restart the app, then send a diagnostics report. Then try if a restart solved the issue 
