Thank you for your suggestion.
Originally I had downloaded the “mt-daapd_svn-1696-1ds101g_powerpc.ipk” file to my Mac and copied it to the DS107e, but now I have downloaded the file using the Download Manager of the DS107e, and this time installation went OK.
I still can not access the Firefly web admin interface at port 3689, so I hope he is still busy indexing. But there might be another problem; the DS107e’s own iTunes server did not work as well so it might be that the original Firefly server encounters the same problem. I wish I knew what is it is…