I see three common problems in http://commons.wikimedia.org/wiki/Category:Tiff_files
1: Thumb of: "Error creating thumbnail: The uploaded file contains errors."
2: Empty thumb with: "Error generating thumbnail
Error creating thumbnail: The resolution of the source file is too large.
No thumbnail will be generated."
3: Thumb of: "Error creating thumbnail: Cannot get metadata from TIFF"
The latter can be solved by Purging the image by hand (dirty/non-existant serialized metadata ?). This last one can probably be fixed easily.