Our custom buttons are added by a script looping through all button and input elements with the class "button" applied. After these elements an image link is created. An event is hooked on this link, which in case of click, pushes the original button, or if it is a submit button, submits the form.
This all worked very well for all forms but one. After too long time spent Googling and debugging, I discovered the solution! Because the original submit button was named "submit" it wouldn't submit the form. So after renaming it, all was good again! I think it behaves this way because the submit element replaces the submit() function of the form.
<!-- BAD --> <input type="submit" name="submit" /> <!-- GOOD --> <input type="submit" name="continueButton" />