To resolve the issue, a reinstall of the primary node must be performed.
Verify that the following jars exist in the /PAM/server/c2o/.c2orepository/.c2oserverresources/lib/ folder:
The versions may be different depending on the version of Process Automation, but the file names will be the same. You may see for instance jetty-websocket-8.1.10.jar instead of jetty-websocket-8.1.19.v20160209.jar - but it's the jetty-websocket that you are looking for.
If these do not exist in this mirroring folder on the primary node, a reinstall of the primary node is required.