Can a document that has been published in TUprints be withdrawn or ‘unpublished’?

No, the publication of the thesis is a pre-requisite for receiving the academic qualification. Moreover, unpublishing’ a thesis is practically impossible due to it having been transmitted to other institutions, including the Deutsche Nationalbibliothek. Sect. 19(2)f. of the TU Darmstadt Publication Service Guidelines also forbids this practice.