WP Smush Pro: Images were skipped due to an error.

I am receiving the following error and was wondering where I can see which files are causing the error(s). It would be ideal is Smush Pro could list the files in error. Is there a log file or similar where I can find out which images are causing the error?

Webmaster, you have 1 image that needs re-compressing!
4 image(s) were skipped due to an error.
Error posting to API: 500 Internal Server Error

Please advise,