Scheduled article isn't post. Failure notification: Scheduling failed

Hi Team,

A client scheduled an article for last night but she got a failure notification: 'Scheduling failed'

=> http://www.screencast.com/t/Q8utiHhVr

This is the specific article:

=> http://elearningmadeeasy.nl/vijf-kostbare-fouten-met-video-die-jij-nu-kunt-vermijden/

Any idea why she got this failed?

And how to prevent this from happening again?

Thanks in advance,

André