In 2017 the Federal Administrative Court ruled on when a superior is too biased to appraise a civil servant. Bias exists when the superior is not willing or not able to judge the official sachlich und gerecht, objectively and fairly.
Tensions that arise from the work itself are, as a rule, not enough. Neither are emotionally colored reactions in a single case, nor single inappropriate, flippant or clumsy formulations.
In this case the court still sent the matter back. The lower court had not examined the superior’s earlier letters and an appraisal from 2006 with harsh wording.