To reproduce, go to the "Settings" tab of any action page in a website.
Add some tags to the page if there aren't any already.
Click on the pagetype specific settings tab (e.g. "Feedback settings", "Petition settings", etc.); the tags that were visible on the Settings, Dashboard, etc tabs will no longer be visible, but the "add tag" button will be visible. If you try to add a tag from this page, you'll receive a red error flash that says "Failed to save tags"
Thanks Jeff - this was reported here - Page tags are not visible in every tab of a page - but your issue mentions the error message so I'm going to keep them separate.
I will let you know if there are any updates!