Going back to 5.5.14 solves the issue. Seems the field type becomes undefined. If we re-add the field we have to manually re-add the images and then it works so it doesn’t seem to be a theme issue.
Hey,
no solution, but we had very similar issues, see here: https://support.advancedcustomfields.com/forums/topic/image-field-in-5-6-0-suddenly-returns-object/
A rollback to 5.5.14 resolved the issue for the time being.