ThreeSixes
Vital Football Hero
If it really is as simple as changing the words from, "Arbeit macht frei", to, "Vaccines are safe path to freedom", then I really don't know what the fuss is about or why he's been arrested. He's making a political point, one that most people will reject, but a political point nevertheless. There doesn't appear to be any hate speech there.
We don't know the reasons for the arrest as you have stated, but people are going to be offended by someone equating the vaccination of a nation to try and make people safe, with the deliberate killing of millions of people. People relate Aushwitz with the gassing of 6 million Jews / Gypsies, not with a tyrannical dictatorship oppressing people.
They could have used other images (e.g the "safe" Titanic), or cartoons of Nazi soldiers etc