When it comes to salary cuts, it is important that they are not arbitrary and at the whim of the boss. Even a bad day at work or a failed PowerPoint presentation is not enough to simply reduce your salary.
The situation is different if you have made gross mistakes at work or have performed poorly or not at all over a longer period of time. In this case, your salary may be reduced - if necessary after consultation with the works council. Another exception is if a company is demonstrably in an economic emergency.