I finally got my testers to get past this SCEP error. Hopefully this works for you too.
Workaround:
- Open the iOS Settings app, select Safari then tap on “Clear History and Website Data”
- Open the Safari app and go to testflightapp.com
- Log into your TestFlight account
- On the “Connect this device by installing profile” screen, tap the green button “Connect Device”
- Tap “Install” on the “Install Profile” screen
- If you have an unlock passcode on your device, you may be will be prompted to enter it
- When the SCEP error comes up, tap “OK”
- Tap “Cancel” on the “Install Profile” screen
- You should then be redirected to TestFlight in Safari with the “Device successfully connected!” message