The file system structure on the disk is corrupt and unusable. Please run the chkdsk utility on the volume NAME_OF_VOLUME.
We started getting this message out of the blue on our main file server a couple of times a day, even though nobody was experiencing any problems accessing files on that volume.
I performed a "chkdsk /f" on the volume after hours during a reboot, which found and fixed minor inconsistencies on the drive.
Since then we have not received any more errors, at least for now.