I now frequently get a strange error while trying to defragment the disk. The doalog displays (actually in French, but I give a translation here) "Defragmentation stops because of insufficient disk space".

This is quite traoublesome, because there is ample enough free space on disk (more than 40 GB).

I ran chkdsk, and there's no error. (note: there's no fragmented file when this occurs, only minor fragmentation of free space, with "0%" fragmentation statistics displayed)

This occurs when the disk is almost completely defragmented, apparently it tries to fill in some very small free gap, and cannot fit any file in it, or Defraggler incorrectly computes the space needed to move a file there. It looks like this is a bug when trying to compute the space needed to perform a file move in a small free gap. I think this can occur with sparse files, or some compressed file, but the dialog provides no debugging info about which file caused that error.

Is there a way to enable some debugging logging (in a log file or in the system event viewer?) when such error dialog occurs, in some hidden option that can be set in "Defraggler.ini" or in the registry?

Here is a screen snapshot under Vista (in 8-bit PNG format)

The same things happens in Windows XP Professional. Only 15% of 60 GB in use, but still not enough space. Strangely enough, when I choose to just defragment the fragmented files it works...

Try using the command-line parameter /debug3. This will create a log into the Defraggler installation path.