I had this problem both with upgrade to 14.8 to 15 and 14.8 to 15.0.1 on iPad 6th Gen. In each case I had the springboard boot loop immediately after upgrade as soon as I input the unlock PIN. Both times, I had to manually downgrade to 14.8 to get it working again and restore from backup. It seemed to me that some setting was causing the loop, so I used "reset all settings" and THEN did the OTA upgrade from 14.8 to 15.0.1 and it worked perfectly. Kind of a pain, but better than having to set everything back up from scratch since no app data is deleted with "reset all settings".
wdsims3
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution