When trying to open a copied file from another project, I got:

An internal error occurred during: "Processing Dirty Regions".

java.lang.NullPointerException
at org.eclipse.wst.sse.ui.internal.reconcile.validator.ValidatorStrategy.endProcessing(ValidatorStrategy.java:140)
at org.eclipse.wst.sse.ui.internal.reconcile.DocumentRegionProcessor.endProcessing(DocumentRegionProcessor.java:71)
at org.eclipse.wst.sse.ui.internal.reconcile.DirtyRegionProcessor.run(DirtyRegionProcessor.java:647)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)

and

"problem with reconciling"

java.lang.NullPointerException
at org.eclipse.wst.sse.ui.internal.reconcile.validator.ValidatorStrategy.reconcile(ValidatorStrategy.java:205)
at org.eclipse.wst.sse.ui.internal.reconcile.DocumentRegionProcessor.process(DocumentRegionProcessor.java:183)
at org.eclipse.wst.sse.ui.internal.reconcile.StructuredRegionProcessor.process(StructuredRegionProcessor.java:252)
at org.eclipse.wst.sse.ui.internal.reconcile.DirtyRegionProcessor.run(DirtyRegionProcessor.java:633)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)