After looking at the only edit made by an IP on Portuguese Wikipedia
and going to the diff of his edit and clicking on "undo", which points to page , MediaWiki displayed the text of [[MediaWiki:undo-success]] but the diff was empty, I mean, there was no changes made.
Could you check why it didn't revert to the original version of the article? (or, alternatively, if it is not able to revert, why didn't show [[MediaWiki:undo-failure]] instead?)
PS: For now, I didn't remove the content, so that you can check what is wrong.