Err 15 is an overheat error code.
If this is shown and you cannot open the door you need to run a test cycle to clear the code and that will allow you to open the door.
Ive uploaded the instructions on running the test cycle so try that and that should open the door.
click here and follow the instructions
But as to the fault once its cleared retry it and fingers crossed it may have been a one off but if it does it again then its either a temperature sensor or control board fault.
Let me know how it goes.