Which SAGA version are you using? I've found this QGIS bug report mentioning the same error as yours. The solution there was provided by Giovanni Manghi in the end of the post:
saga 2.0.8 compatibility check is not enabled.
The fact you see this option suggest me that you have an old version of 'Processing' masking the latest one shipped with qgis 2.8, because it does not exist anymore.
Check inside .qgis/python/plugins
If you have a "processing" folder you must delete it. Then restart qgis.
