A bug report has been logged with the developers and the issue should hopefully by fixed in the next version.
Unfortunately, I do not know when the next version of SimpleViewer-Pro will be released.
If you would like to be notified when the next version is released, please follow us on Twitter @simpleviewer.
In the meantime, all you need to do (to avoid having to manually edit the gallery's XML file) is, after selecting 'Show Back Button', click in the 'Back Button Text' field and then leave the field by clicking in another field (or click the 'Refresh' button).
SimpleViewer Support Team