X11 error message unter shiny server but normal user works

I'm getting an error when I run my shiny app. It says

Warning in pngfun(filename = filename, width = width, height = height, res = res,  :
  unable to open connection to X11 display ''
Warning: Error in .External2: unable to start device PNG
  127: pngfun
  126: startPNG
  125: drawPlot
  111: <reactive:plotObj>
   95: drawReactive
   82: origRenderFunc
   81: output$GB
    1: runApp

I searched already a lot of examples but somehow this is a little new. I managed to get everything running as normal user and checked the capabilities()

> capabilities()
       jpeg         png        tiff       tcltk         X11        aqua 
       TRUE        TRUE        TRUE       FALSE        TRUE       FALSE 
   http/ftp     sockets      libxml        fifo      cledit       iconv 
       TRUE        TRUE        TRUE        TRUE        TRUE        TRUE 
        NLS     profmem       cairo         ICU long.double     libcurl 
       TRUE       FALSE        TRUE        TRUE        TRUE        TRUE 

However, when I su - shiny the capabilities change to

> capabilities()
       jpeg         png        tiff       tcltk         X11        aqua 
       TRUE        TRUE        TRUE       FALSE       FALSE       FALSE 
   http/ftp     sockets      libxml        fifo      cledit       iconv 
       TRUE        TRUE        TRUE        TRUE        TRUE        TRUE 
        NLS     profmem       cairo         ICU long.double     libcurl 
       TRUE       FALSE        TRUE        TRUE        TRUE        TRUE 

So X11 looks unsupported for my shiny user and that might be the problem within the shiny server? I have a debian linux system and I guess everything is installed, since it is working as a normal user. But not sure how to check and what could be missing.

Hope I can get some hints for the trouble shooting...