Steps to reproduce:

1. Create a page template and enter some contents.
2. Save the page template
3. Click "Edit" so the edit form opens, and let your session expire (or clear the browser's session).
4. When your session has expired, return to the browser window (still showing the template contents in a textarea) and hit "Save", "Preview" or "Cancel" in the edit section.
5. You will be redirected to login, so log back in.
6. The contents of the template is now missing

In the table PAGETEMPLATES of Confluence database, a row of a new version of the page template has been created. However, the value for field content of that row is empty.