Frame title meddles with text area
Adding a title seems to shift the text are up, causing it to overlap with the frame header. IIRC, @saak told me that standard/default height of the header is determined based on the title page. This basically means that every content page is an exception to that default. I would suggest to instead have the title page be the exception.
\documentclass{beamer}
\usetheme{mpi2015}
\begin{document}
\frame{\rule{1cm}{\textheight}\the\textheight} % ok
\frame{\frametitle{Not ok}\rule{1cm}{\textheight}\the\textheight}
\end{document}