I tried to add a tag 'aa-gun' to one of my recent places but it came back with an error, as captured. It would seem it now only accepted tag items already in the system and not new tags. I managed to add 'Tai Mo Shan' but it was displayed as 'Tai Mo Shan (2728)', not exactly what I typed.
I confirm the problem. At the moment we can only add existing tags, it will not automatically add new tags. (In this case a workaround is to use the existing tag anti-aircraft gun). Also when adding a tag the site adds the tags ID number after it, eg Tai Mo Shan (2728). The number only appears while editing, and after saving it is shown as just Tai Mo Shan.
I'll add this to the list of things to check.