Steps to reproduce:
1. Navigate to Site Administration > Site Content > Web Content.
2. Create new basic web content called test.
3. Navigate to Site Administration > Site Content > Tags.
4. Create a Tag called tag.
5. Edit the test Web Content and associate it with the Tag created in Step 4.
6. Publish the Web Content.
Adding tag to the web content successfully.
Unable to add tag to the web content, error message shows up on the page.