mktextfm ecss0400 fails

The beamer class, more precisely beamerbasefont.sty, has its own \DeclareFontShape for the Computer Modern/European Modern fonts.

The declarations are disputable, as the standard don't dare to choose fonts with design size below 5pt (below this threshold fonts are scaled). Compare with fix-cm.sty that does

\DeclareFontShape{T1}{cmss}{m}{n}{
        <-9>    ecss0800
        <9-10>  ecss0900
        <10-12> ecss1000
        <12-17> ecss1200
        <17->   ecss1728
      }{}

whereas beamerbasefont.sty has

\def\EC@family#1#2#3#4#5{%
  \DeclareFontShape{#1}{#2}{#3}{#4}%
  {<3><4><5><6><7><8><9><10><10.95><12><14.4>%
   <17.28><20.74><24.88><29.86><35.83><42.99><51.59>genb*#5}{}}

\EC@family{T1}{cmss}{m}{n}{ecss}

which is the culprit: when \Tiny is processed, which chooses a base font size of 4pt, TeX needs to generate ecss0400, which is not a tested design size and, indeed, its creation fails.

The package beamerfontsize.sty should be fixed not to use such small design sizes and to use instead fix-cm.sty.

Workaround: \usepackage{lmodern}.