We had this issue while we did 18.104.22.168 upgrade last week. It was a hectic weekend, but with help of my patner Apps DBA we made it easy and delivered the instance 11hrs before actual outage. So come to the issue. I have mentioned in of the my previous Post regarding Xvfb setup in our production env admin node. Display variable is working fine adnd we dint' have any issues with DISPLAY, but getting "BIB-7000 Cannot access X Server" message while accessing graphs in Workflow pages from OAM.
While investigating realized this error was coming from Application tier where we do not have setting for Display, or I can say was set to somewhere else which was not working. As we didn't want to run Autoconfig again, what we did. We changed DISPLAY variable in jserv.properties file to our admin node Display setting and bounce Apache. Vola it worked.
Happy Troubleshooting !!!