1. Close Collector for ArcGIS.2. Connect the device to a PC via USB.3. Using Windows Explorer to access the device, navigate to ArcGIS_Collector > offline_data > basemaps.4. Cut and paste any offline basemaps into a folder on the PC.5. Using a process of elimination, paste each basemap folder back into the 'basemaps' folder on the device, and open Collector for ArcGIS between each transfer. 6. Once the problematic basemap(s) have been identified, remove them from the device and re-download if necessary.