It seems that you've got the answer for this question through href="https://support.atlassian.com%20">https://support.atlassian.com and the solution for this was remove one of the duplicate .jar file having older version and restarting Confluence.
Don't worry to ask again, if you nmeed some additional help.
All the best,
I have seen similar problems when using incompatible plugins or having problems with the plugin cache.
First try clearing your plugin cache
If this does not solve the problem go through your plugin list and update them to compatible versions with your new Confluence version.
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