If gparted detects a filesystem to be of a different type to that indicated in
the MBR, it refuses to let you alter it.  However, it doesn't tell you why you
can't alter it.  With 0.2.3 the reported error is that the filesystem doesn't
exist; with 0.2.2-5 the error is that gparted can't read the filesystem.  The
situation would be much easier to diagnose if gparted instead said that the
detected filesystem type does not match the type in the MBR.

Other information:
This problem came to light when trying to resize an NTFS volume.  Gparted
refused to alter it, and we were left a bit stuck.  It turned out that the disk
had been marked as dynamic by windows, even though it only contained one
partition.  Windows seems to mark dynamic disks by setting the partition types
to 42 i.e. SFS.   However, the partition type remains as NTFS - I guess
Microsoft don't care that they are abusing an assigned partition type.  Once we
realised what the problem was, we used fdisk to change the type back to 7 (NTFS)
and then refreshed the disks in gparted, at which point everything went
smoothly.  If gparted had reported the error in the first place we'd have been
done in a few minutes.  As it was, it took quite a bit longer!