Hi! The upload failures are caused by underlying problems in the ArcGIS Online infrastructure. Esri Technical Support is handling this under this incident:
INC-000402 Uploading multiple images to hosted feature services intermittently fails with a 500 error
Note that this is affecting Survey123 as well as other ArcGIS apps such Map Viewer, Experience Builder and others. To get the latest status on the resolution to this ArcGIS Online problem, please contact Esri Tech Support and have them associate your customer account number to the incident above.
Many thanks for raising this issue. I have shared with the Online team this thread.