You can look in your orion.log for errors on your epo server but I'm assuming an upgrade went bad right before you got this.
run the mertool on your epo server and open an SR with McAfee. This is not easily fixed typically. Extensions have to match version in 3 spots and you better play that matching game with mcafee at the other end.
See this kb
Before you do any of that stuff grab a backup of the epo directory on the epo server and a separate db backup
At the end we have reinstalled ePO Console as described in KB66616
With one exeption, we used new folder "..\McAfee\ePolicy Orchestrator\Server\conf\Catalina\" where we copied missing files from old folder, in particular DATALOSS2000.xml
This actions have solved our problem and now we again can login to ePO console without any errors