Hello Julian,
I don't have first-hand experience with the Hauptwerk v5+ version of Sonus Paradisi's Zwolle sample set, but based on the filename in the error message it would appear to be a 'multi-part' package. Assuming so, all of the parts need to be present in the same folder, and all of the parts need to have exactly the correct filenames. The filename you mentioned:
ZwolleSurround.part01(5)CompPkg_Hauptwerk_rar
... looks wrong to me -- I would expect that the file should instead be named exactly:
ZwolleSurround.part01.CompPkg_Hauptwerk_rar
Probably your download manager changed the name/extension [by inserting the '(5)' and perhaps removing the second dot character] when you tried to download it after an initial failed download.
I.e. I would expect that the files need to be named exactly as follows:
ZwolleSurround.part01.CompPkg_Hauptwerk_rar
ZwolleSurround.part02.CompPkg_Hauptwerk_rar
ZwolleSurround.part03.CompPkg_Hauptwerk_rar
...
ZwolleSurround.partNN.CompPkg_Hauptwerk_rar
... where 'NN' is the two-digit number of the last part.
Does renaming them accordingly solve it?