Did you try to add the photos directly to Lightroom or to a folder in Files on the iPad? As a test - Try copying just one photo from the SD card to a folder in the Files location and then add to Lightroom from there.
Do you see the same error when you first import one of the failed photos to the iOS Photos-app and the manually import from within Lr Mobile. Just trying to understand if this is related to the direct-sd-import or connected to the general import engine.
Hope to get a clue about your issue soon. - Guido/ Lr Mobile QE