Optional field is required during validation of contenttype

Tested with:
Drupal: 7.20
PHP: 5.3.14
Webserver: Apache
Modules enabled: Media, Jwplayer, Ctools, libraries.
Language: dutch, first day: monday, Timezone: Europe/Berlin

Created a content type: video.
Added 2 rijksvideo widget/field.
Added taxonomy term reference, set required
Fields are set "not required"
Created new page
Left videofields blank.
Taxonomy term filled in.
save
Edit new page
Videofields are still left blank.
Removed taxonomy term.
Hit save
Form validation sayes:
Taxonomy term field is required.
You need to add an MP4 file.
You need to add an MP4 file.

This is strange because drupal sets a value in field: Enter the paths of the video files manually

While I didn't do this. And the videofields are optional.