I already re-installed onto a new server. I was only testing .. If I do reproduce I will make sure to provide stacktrace. I do appreciate you looking into this. I will make sure in the future that I can provide more details.
@Suttilak-Wattanapaiboonsuk There's an autocomplete when you write a tag.
What I've done on my forum is that I created a subject in an hidden category where I create each tag I want to appear in ACP (to color them) because tags with 0 uses are still visible and themable. You are limited to 5 tags by default but you can change them just need to submit post to keep the tags 😉