After you have installed CMIS-compliant or SharePoint repository and configured it in the Primavera P6 Administrator, you will need to do the following to ensure the content repository works with WebSphere 8.5:
Stop running WebSphere.
Go to websphere home\AppServer\profiles\profile name\installedApps\cell name\Primavera.ear\p6.war\WEB-INF\lib.
Copy jaxb-impl-2.1.11.jar.
Paste the jaxb-impl-2.1.11.jar in the "websphere home\AppServer\lib" folder.