At the Commons, there have been two recent error reports about thumbnails not being generated. The two files mentioned are
File:Mount Everest 1980 - Norbu+Heinrich.jpg
The second one has in the meantime been moved to
File:Mount Everest 1980 - Norbu and Heinrich.jpg
which resolves the problem.
See also bug 8045, which was marked as "fixed" in 2007, but apparently without anybody knowing what "fixed" it.
Evidently this needs re-fixing.