I've finally found the solution!
Edit /etc/ntlmaps/server.cfg and remove (or comment) following lines:
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-excel, application/msword, application/vnd.ms-powerpoint, */*
User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows 98)
2009-10-30 12:59:57 UTC in NTLM Authorization Proxy Server
