Problem/Motivation
When filtering by taxonomy term, the Operator "Is one of", "Leave blank for all" is not working. You cannot save the view and it shows "No valid values found on filter: Content: (taxonomy vocab name)"
Steps to reproduce
In a default Drupal installation, we should have taxonomy vocab "Tags", and content type "Article" with field_tags.
1. Create 2 terms under the taxonomy vocab "Tags", e.g. 'tag1', 'tag2'
2. Create a new view, by default it has filter "Content: Published (= Yes)"
3. Add another filter "Tags (field_tags)"
4. On first pop modal. Under "Vocabulary", choose "Tags". Under "Selection type", choose "Dropdown"
5. On second pop modal. Under the "Operator", choose "Is one of". Under "Select terms from vocabulary Tags", do not select anything. i.e. as the description suggested "Leave blank for all"
6. "Apply"
7. You will not able to save this view.
Proposed resolution
In order to save this view, you have to select all terms in step 5 above.