This problem happens because you are using the latest version of SimpleViewer (v2.3.1) but your gallery's XML file is in an older format (v1.9) which the Flash Player still understands but the Mobile Player (which was introduced in v2.1) does not.
The image entries in your gallery's XML file look like:
... but the v2.x format would look something like:
<image imageURL="images/DSCN1117.JPG" thumbURL="thumbs/DSCN1117.JPG" linkURL="" linkTarget="">
You should be able to convert a SimpleViewer v1.9 gallery to the latest version by following the 'Upgrading Existing Galleries' instructions on this web page.
SimpleViewer Support Team