There's a problem described in the following bug report: https://jira.atlassian.com/browse/CONF-28111

On most databases the only symptom is just having a null value entry on the sidebar, however when reproducing the same problem with an instance running on Oracle, the sidebar is not configurable anymore, only a spinning wheel is shown that never loads.

Steps to reproduce

1. Install Confluence with Oracle DB backend
2. Create two spaces
3. In one of the spaces click Configure Sidebar
4. Add a new entry, select Search and search for the other space
5. Select the space itself (not the space homepage)
6. Notice that an entry is added as either 'undefined' or 'null'
7. Try editing the sidebar again, notice that it is not working

Workaround
Open a support ticket at http://support.atlassian.com to verify which entries are to be deleted from the AO_187CCC_SIDEBAR_LINK database table. You should only do it after confirmation, but you should look for entries with EXTERNAL_LINK type and for those that belong to the offending space key and have an internal URL of your instance defined. The mentioned rows should be only deleted while Confluence is not running.

Solution
Do not use space linking and wait for the solution in https://jira.atlassian.com/browse/CONF-28111 to fix this functionality