@PhilipKroon: You have to write it as
Note that the global variable libname is an expression sequence containing the possible library paths (as defined for instance in the maple.ini file). If several folders exist, then you can also choose , , etc., above, but the choice should of course be consistent with the library folder in which you have put your MineRedskaber.mla.
Or, coming to think of it, if you do not want to use libname but rather use an explicit path, as seems to be case, then write it as
LibraryTools:-Save(MineRedskaber,cat("C:/Program Files/Maple 2017/lib","/MineRedskaber.mla"));
i.e., drop the selector , and insert an appropiate root, C:/ say; or, what amounts to the same,
LibraryTools:-Save(MineRedskaber,"C:/Program Files/Maple 2017/lib/MineRedskaber.mla");