If directory name for todo.txt in Dropbox contains non-ASCI symbols ("\сделать" in my case) then while archiving "\%D1%81%D0%B4%D0%B5%D0%BB%D0%B0%D1%82%D1%8C" directory is created.
"\сделать" dir is created by official Android Todotxt app by default for my locale (RU);
"\%D1%81%D0%B4%D0%B5%D0%BB%D0%B0%D1%82%D1%8C" is url-encoded "\сделать" (UTF-8)
If directory name for todo.txt in Dropbox contains non-ASCI symbols ("\сделать" in my case) then while archiving "\%D1%81%D0%B4%D0%B5%D0%BB%D0%B0%D1%82%D1%8C" directory is created.