I'm suffering the error explained in this question of Stackoverflow.
Summarizing, when validating an xhtml or an entire dynamic webproject a window error appear with the error:
"An internal error occurred during: "Validating".
Could not initialize class org.apache.myfaces.shared.config.MyfacesConfig"
In Stackoverflow they correct a MANIFEST.MF file located inside a jar (I think in Neon must be plugins/oracle.eclipse.tools.webtier.jsf.facelet_15.7.1.201606271248.jar)
The problem is the solution explained in Stackoverflow doesn't solve my error.
I have also tried to edit manually the MANIFEST.MF of the jar and delete the text ".tests" of all packages.
I'm using:
-oepe-12.2.1.3-neon-distro-linux-gtk-x86_64 (Eclipse Version: Neon (4.6) Build id: I20160606-1100, oepe plugin version 15.7.1.201606271248)
-Java 1.8.0_65-b17
In the past I had the same problem with Eclipse Mars