I'm surprised that the test of creating a secure link to a site file in the HTML editor did not produce a similar error but provided that you are sure that the DNN installation is running version 7.2.2 (or 7.2.1), the cryptography sections introduced in v 7.2.1 should have been automatically merged into web.config during an upgrade from an earlier version or on a new install of 7.2.1 or 7.2.2.
If you have superuser (host) access to the website you can use the Host --> Configuration Manager to merge these changes into the existing web.config file:
1. Verify that the site's root folder has been granted full file system permissions (including modify) for the ASP.Net worker process account (IIS Identity account) and that the web.config file is not marked as read-only.
2. Locate the file 07.02.01.config in either the site's Install/Config/ folder or in the same folder of a newly downloaded and extracted DNN 07.02.01 or 07.02.01 install or update package. Place the file on your local computer were you can easily find it during the next step.
3. Open the Host --> Configuration Manager and select the Merge Scripts tab.
4. Click on the Select File button and select the 07.02.01.config file residing on your local computer.
5. Click on the Upload button. The merge script for the cryptography sections will appear in the editing window.
6. Without making any changes to the merge script, click the Execute Script button.
7. The changes should be merged into web.config and the site's application pool will take 20-30 seconds to recycle.
8. Test to see if this has resolved the issue. Most likely you will need to recreate the links.