Can my employer issue a poor work reference upon my departure?

No — German law (Section 109 GewO) stipulates that a work reference (Arbeitszeugnis) must be benevolent and truthful. This sounds contradictory, but it is not: the reference may not contain wording that harms the employee, even if it would correspond to the truth. There is a coded language that HR professionals know — phrases like 'always tried' signal weakness, 'to our full satisfaction' is standard, 'to our fullest satisfaction' is the top grade. You have the right to challenge the reference if it contains negative coded phrases — at the Anti-Discrimination Office or before the labour court. GDPR: references are personal data — you have the right of access.