Eclipse Issue "An internal error occurred during: "Items filtering". Class file name must end with .class"

Deleting the corrupted search index which is explained in Eclipse bug’s #269820 comment.

How to delete the search index:

1. Close Eclipse
2. Delete workspace/.metadata/.plugins/org.eclipse.jdt.core/*.index
3. Delete workspace/.metadata/.plugins/org.eclipse.jdt.core/savedIndexNames.txt
4. Start Eclipse again

This fixed the issue for me.