It's in the 3MF system. I suspect that if you loaded a .3MF file from an umlaut path it wouldn't load.

@Samir55

I will take a look, LoH .

If that's the case, make sure you use boost::nowide like we do for the other IO:: classes

boost::nowide is used in reading the extracted files from 3mf but the problem is that miniz doesn't extract the file at the first place. I am looking for an easy fix.

@lordofhyphens @Samir55
Problem isn't in miniz as I've tested it by itself and it can work with a string like this "Geräte" in it's path.
I think it's in the 23_3mf.t perl file I've fixed it by decoding the input path and output path to utf8 using Encode::decode_utf8() from Encode, and it's working properly.
But I'm not sure where else should I add this fix beside this test case as it will cause a problem every time read_tmf is called with special characters like 'ä'.
Considering this is a perl side issue it should be fixed before calling Slic3r::IO::TMF::read(input_file, THIS) which is in c++.