Smush Pro - Invalid file format error

Smush Pro gave me 5 error messages after bulk smushing. Two of them tell me exactly which file was problematic. But three of them say this:

Invalid file format. Only PNG, JPEG, and GIF files are supported.

There is no indication of what the filename is, so I can't track down the problem.

Am I missing something?

Support access is on if you need to have a look. Thanks!