Users cannot add labels to Page Templates

Users without the confluence-administrators membership cannot add labels when editing existing Page templates.

Steps to reproduce:
1. log in as a regular user;
2. edit an existing Page Template;
3. click Labels - an error message will be displayed

4. start typing an existing label - it will be recognized

5. select the label from the autocomplete dropdown - another error will occur

Notes:

• this behaviour isn't changed by any of the Confluence Global or Space Permissions;
• no errors occur when adding labels to a new Template.

As a result, users would need help from Support whenever they want to add a label to a Template.