Using latest dev version (released today 2016-Oct-05), however I think this error started before today's release.
Attempting to remove images using the Remove button now generates an ajax error and logs the following:
Notice: Undefined offset: -1 in Drupal\entity_browser\Plugin\Field\FieldWidget\EntityReferenceBrowserWidget->formElementEntities() (line 614 of /modules/contrib/entity_browser/src/Plugin/Field/FieldWidget/EntityReferenceBrowserWidget.php).
Thanks @joshua for providing a patch!
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.