I've run into a bug with the newest version of K2 (2.6.8). If Force SSL (https) is on for the admin area the auto complete in the tagging section doesn't work (it just spins). Oddly, the Additional Categories section at the bottom works fine and auto completes. Does anyone have some suggestions how this could be fixed? I'd like to run the admin in https for security reasons, but its a bugger not to be able to tag. Thanks for any help.

Joomla 3.3.3
K2 2.6.8

Are you getting any js errors?

Chrome can block files if they are not from a secure (non-https) source if you are using SSL.
Have you tried it with another browser?

Krikor. I have tried it with Chrome, Safari and Firefox. In all 3 it doesn't work. I have eliminated anything in my htaccess causing it and I'm not seeing any Javascript errors showing up. The only error I'm seeing is this.

[Warning] The page at website.com/administrator/index.php?option=com_k2&view=item&cid=1239 displayed insecure content from website.com/index.php?option=com_k2&view=item&task=tags. (index.php, line 0)

Could you try installing the DEV version?
You can get it from here: getk2.org/assets/get/

PS. Backup first.