I'm going to answer this question based on my understanding of copyright law in the United Kingdom. The general principles are likely to be the same in German law, however, for a definitive answer, you should consult an expert in German copyright law. I am not a lawyer. This is not legal advice in any jurisdiction.
The author of a literary work - in this case your presentation - is usually the first owner of the copyright in that work.
However, if that work was created in the course of employment, then the copyright in the work belongs to the employer.
The phrase in the course of employment requires careful consideration in such cases. I assume that you are an employee of your university, and that you were employed to conduct research. I suspect that you were also expected, as part of your employment, to create presentations of your research. I assume that these expectations were put in a contract of service - the usual case for a university researcher. This is distinct from obligations under a contract for service - which would be more common if you were commissioned to produce a specific piece of work.
If you were employed to conduct and report research as part of a contract of employment - the most likely case given what I know of your situation - it would appear that any copyright in your presentation rests with your employer, the University.
You mention co-authors. I assume that these are co-authors of the publication(s) regarding the research which you are describing in your presentation. If this is the case, and your co-authors did not contribute to the creation of the presentation itself, then the copyright in the presentation rests with solely you as first author or, by virtue of employment, solely with your employer (the most likely situation, as I read it).
If, on the other hand, your co-authors contributed to the creation of the presentation itself, then they - or their relevant employers - may be entitled to be considered joint owners of copyright in the presentation.
I have been discussing the copyright of the presentation - not the copyright arising in any research publication. I assume that you have included in your presentation items such as pictures, plots or graphs which appeared in your research publication. It is most likely that copyright for these works vests with the publisher of the research article.
If this is the case - that your research has been published (including, for example a graph) and that the copyright in that publication has been assigned to the publisher (again, the usual situation), then the use of that work in another work - e.g. using a graph from that publication in your presentation - may be considered an infringement of the copyright in the publication.
There is an exception to copyright infringement in several copyright laws which permits the use of copyrighted works for educational or research purposes. In this case, your University may well lay claim to copyright in your presentation as a literary work by virtue of their employment of you, however, they may also have to consider that they are able to use the copyrighted works included in that presentation (the plots, graphs etc, the copyright of which is held by the publisher of those works, e.g. the research journal) if the use is for research or teaching purposes. If, for example, the University decides to include your presentation - including a plot published in your original research paper - in a book, and then sold that book, the University is likely to require a licence from the journal which holds the copyright in the plot.