Hi Björn,
the issue seems to be with the libraries - I also had the Problem, and was able to fix it with the reinstall of QGIS.
Install Qgis with the OSGEO network installer as described in their wiki on github. (I installed it with all libraries - not sure if installing only the ones that they recommend is enough in general?)
Check out this thread on github where they discussed it.
github.com/ArteliaTelemac/PostTelemac/issues/7