After upgrading to 2.10.x some pages may not render correctly, and will throw an error similar to this:

Cause

javax.servlet.ServletException: Servlet execution threw an exception
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:313)

caused by: java.lang.NoClassDefFoundError: com/atlassian/confluence/renderer/WikiStyleRenderer
at java.lang.Class.getDeclaredMethods0(Native Method)

caused by: java.lang.ClassNotFoundException: com.atlassian.confluence.renderer.WikiStyleRenderer
at java.lang.ClassLoader.findClass(ClassLoader.java:359)

It appears that the upgrade does not remove the older version of the Page Tree plugin, this can be confirmed by checking the list of installed plugins:

• Page Tree (com.atlassian.confluence.plugins.pagetree, Version: 1.10.1)
• Page Tree (bnpparibas.confluence.pagetree, Version: 1.3)

Removing the older version of the Page Tree plugin will resolve the errors.