1. Open your mc launcher.
2. Press "Edit Profil" in the lower left coner.
3. The Profile Editor will open.
4. Add a hook in the box Launcher Visibility.
5. Change the drop down menu to "Keep the launcher open".
6. Press "Save Profile" to save the changes.
7. You will return to the main menu after saving, press "Play" to launch the game.
8. The game will launch normal, except that the launcher is not closing. Enter your game and reproduce the error.
9. After doing this, press the "Development Console" tab at your launcher.
10. You will see the log file about all actions that where happening while you are playing. Lines with [INFO] [STDERR] containing error messages.
11. Right click into the text with your mouse and select "Copy all Text"
12. Paste this text into your posting.
13. Optional: Undo the changes in the launcher to the original settings