Dear Enrico,
Upon opening the included .msl-file in wwtp.base.msl (wwtp.VolumeASMConversionModel.body.msl) and expanding all files this includes in the Includes tab, no red 'X's are shown. I have no idea what file would be missing, but I did notice (as you can see in one of the previously attached figures), that the "no such file or directory" is thrown for KOSIM.BlockLib.xml..cpp.msl, i.e. a file with two dots between xml and cpp.
I was also surprised to see an absolute path in the model library, which doesn't seem like good practice from my predecessors, as it limits the reuse possibilities of the library. This concerns the model library WESTforKOSIM that is used for courses on integrated modelling at BIOMATH.
Could this maybe be due to upgrading to WEST2016?
I hope this provides some more information.