The best option is to send a mail from the game itself; first go to the main menu of the game, press the wheel in the top right corner to open the settings, and then press support. This will give us additional information about your account that can be very useful to resolve bugs.
Bugs can also be reported by sending them to email@example.com or by creating a topic on the forums.
When reporting bugs, please be as specific as possible about what went wrong. If possible, add screenshots and explain the steps you did that resulted in the bug occurring.