I'm far from being a tax expert but being in this business for 18+ years I would say that your positioning airport is not the deciding factor in determining tax liability.
Deciding factors are, among others I'm sure, principal main office where company is based, where your family reside and go to school, work and live their lives and also where you spend most of your days over a year.
For example, if you position in/out of SXF, but you and your family live in Poland, I find it hard to believe that you would be tax liable in Germany. All you are doing is using an airport in Germany to get to and from work, that can hardly qualify for German tax authorities to claim rights to your taxes?
Like I said, I'm certainly no tax expert so I'd be happy to be told otherwise with regards to this subject.