polish letter Ł and ł are replaced by underscore in filenames. They should be replaced by L and l. All the other polish letters ąćęóńźż are replaced by their corresponding unaccented characters. All of them are in lookup tables in inc/Utf8/tables, including Ł and ł.
This is similar to problem described here: https://bugs.dokuwiki.org/164.html but that seems to long be fixed.
Ł is 0x0141
ł is 0x0142