If it's the frontend, it's probably a JS issue coming from the site's template or some system plugin being loaded in the background. You'll have to provide more info. If you need us to have a look directly, use our contact form to send us temporary user access to the site with instructions on how to get to the K2 item form.

If you can't do that, open up Dev Tools in your browser and switch to the Console tab. Reload the item form and check for any errors (in red). Paste them back here. The errors may trigger only when you attempt to add a tag, not just loading the K2 item form in the frontend.

Can you install the dev version please?

github.com/getk2/k2/archive/master.zip

Check again and if the issue persists, let me know.

First look carefully for differences in "system" plugins. These types of plugins can load CSS/JS on any URL in Joomla. If you have template overrides for K2 in your template, locate and delete the file itemform.php (but only inside your template's /html/com_k2/ folder - and if it exists of course).

You're welcome :)

So it was some old leftover override. Sometimes template devs just copy the entire templates folder in K2 even if they don't use some file...