Note that '?', U+03B1 GREEK SMALL LETTER ALPHA, is encoded as two bytes CE,B1 in UTF-8, and 'α' is what you get if you interpret that as Windows-1252 (English), it looks like CE,B1.
Now the plug-in might not be interpreting it the same way as the dialog box. It's easy to miss things in error reporting, or leave problems like using -A forms of functions that are done correctly in "meaningful" code.
Checking the underlying call to CreateFile, I see both:
Thumbs9.exe 4692 CreateFile \\OORT\Media\Images\Days\2015\05-May\?6000 camera practice\DSC00002.ARW SUCCESS
Thumbs9.exe 4692 CreateFile \\OORT\Media\Images\Days\2015\05-May\α6000 camera practice\DSC00002.ARW NAME NOT FOUND
(aside: this forum doesn't have
formatting! )
where the first (good) one comes directly from Thumbs9.exe and the latter (bad) one comes from PolyImagePro.DLL.
So you are calling functions with UTF-8 strings when they expect so-called ANSI strings, or transcoding current-code-page-to-wide before calling -W functions, or some permutation on that.
Changing to use DCRAW plug-in instead, there are no errors reported but thumbnails are not generated. Procmon shows otherwise:
Thumbs9.exe 4692 CreateFile \\OORT\Media\Images\Days\2015\05-May\a6000 camera practice\DSC00002.ARW NAME NOT FOUND
Look carefully: the file name has been changed from '?6000' to 'a6000'.
This call is made from PolyImagePro.DLL via plug_digicam.tpp.
Naturally, changing the file name is not a proper thing to do when loading a file! Not reporting the error is bad on top of that.