You may be found to have defamed someone if you go around stating your opinions as facts and the court disagrees that they are facts.
For example, what if Alice thinks her employee Bob is dishonest, but he hasn’t been convicted of theft? Can she transfer him to a job that does not involve handling money or procurement?
I think, “yes.” Alice can decide that Bob is not a good fit for a job involving money and trust. Bob might then sue for “constructive dismissal” if the new position is not commensurate with his experience.
The lawyers are the only winners in most such lawsuits, so perhaps Alice settles with Bob. He quits and receives severance.
Bob then applies for a job with Carol. If Alice claims Bob was dishonest, Bob might sue Alice and win substantial damages.
Now what about Carol? What if she questions Bob about Alice’s clams and does not believe his explanation of events? Or if a number of people come forward during the ruckus to tell how they feel Bob was dishonest with them?
“Presumption of Innocence” for Carol would oblige her to hire Bob anyways: It means treating Bob as innocent of the accusations in every way until he is convicted in court.
I claim she has no such obligation. Perhaps it is imprudent of Alice to accuse Bob of dishonesty without being able to prove it, but nevertheless Carol has a right to decide for herself whether she wishes to hire Bob based on her own judgment.
tl;dr:
“Presumption of innocence” is a very strict standard that only applies to the state. People have a right to decide for themselves whether people are innocent or guilty, but just as you say, some actions based on those decisions come with tradeoffs and consequences.
At the most; a prudent judge would likely allow it while the case is still being tried. You'd have to deal with defamation only after the completion of the criminal probe, where a suit could be levied under the auspices of "we were deemed innocent".
IANAL, and this is assuming American court procedure; things may be different in Germany.