aagh!
non potevo sopportare questa soluzione obrobbriosa…
questo va nel preambolo dopo aver caricato il tema
`\makeatletter
\setbeamertemplate{headline}
{%
\pgfuseshading{beamer@headfade}\vskip-1.25cm
\ifnum\thepage=1 {%
\begin{beamercolorbox}[colsep=1.5pt]{upper separation line head}
\end{beamercolorbox}
\begin{beamercolorbox}{section in head/foot}
\vskip2pt\phantom{\insertnavigation{\paperwidth}}\vskip2pt
\end{beamercolorbox}%
\ifbeamer@theme@subsection%
\begin{beamercolorbox}[colsep=1.5pt]{middle separation line head}
\end{beamercolorbox}
\begin{beamercolorbox}[ht=2.5ex,dp=1.125ex,%
leftskip=.3cm,rightskip=.3cm plus1fil]{subsection in head/foot}
\usebeamerfont{subsection in head/foot}\insertsubsectionhead
\end{beamercolorbox}%
\fi%
\begin{beamercolorbox}[colsep=1.5pt]{lower separation line head}
\end{beamercolorbox}
}%
\else {%
\begin{beamercolorbox}[colsep=1.5pt]{upper separation line head}
\end{beamercolorbox}
\begin{beamercolorbox}{section in head/foot}
\vskip2pt\insertnavigation{\paperwidth}\vskip2pt
\end{beamercolorbox}%
\ifbeamer@theme@subsection%
\begin{beamercolorbox}[colsep=1.5pt]{middle separation line head}
\end{beamercolorbox}
\begin{beamercolorbox}[ht=2.5ex,dp=1.125ex,%
leftskip=.3cm,rightskip=.3cm plus1fil]{subsection in head/foot}
\usebeamerfont{subsection in head/foot}\insertsubsectionhead
\end{beamercolorbox}%
\fi%
\begin{beamercolorbox}[colsep=1.5pt]{lower separation line head}
\end{beamercolorbox}
}
\fi
}`