How to get "Add another item" option for fields based on taxonomy terms
Problem solved by installation of the Taxonomy Single Tag module and the patch from #6 of this thread. Answer provided by "Poieo" on drupal.org. I have applied this and it is working as desired. However I noticed that without the patch, I was getting errors not only with the new field where the "Single tag autocomplete text field" widget was applied, but also with other fields allowing multiple-item adding – even though they were neither autocomplete nor based on taxonomy terms.