Apparently, my contour map can only be projected in WGS 84 UTM Zone 27N. WGS 84 projection is not working.
Other than that, my three DEMs were in different formats and spatial resolution. I resampled them, but changing formats result in different elevation values. It is very important to have them right since I am looking at volumetric differences of glaciers through geodetic mass balance method. According to the volume calculations using 'Surface volume' tool, my 2004 DEM shows an increase in volume compared to the 1986DEM. It is unlikely that the glacier has grown since, as ice is known to be retreating rapidly in that region. It bugs me that when changing formats and/or pixel type the elevation values and volue changes. It makes me wonder what the actual elevation values are for my DEMS.