In November 2009 Google introduced the new AdSense interface and we are now well over two-years into it where I believe all publishers have been migrated but we still have migration issues from the old interface to the new.
One of those issues are old legacy reports from the old interface.
Google AdSense Engineer Guillaume said they hope to wrap up those last issues by the end of this month. He responded to a complaint that there are these old reports in the new interface that are not usable, nor can they be deleted. Guillaume said in a Google AdSense Help thread:
We aware of the issue; it's an unfortunate side-effect of the migration of saved reports from the old to the new interface. The migration is taking longer than initially planned, precisely due to some old reports not having exact equivalents in the new interface. We're hoping to complete the migration soon, hopefully by the end of this month.
Google is known for giving timelines and missing them. One specific example is the Google Apps and Google+ migration issues that was promised months ago and is still lingering on.
So if you have these AdSense issues - hopefully Google will hit this target and your reporting issues will be resolved by the end of this month.
Forum discussion at Google AdSense Help.