Hi Bjorn,
Is the customer a cloud tenant?
The App Center App has Secure Email detection & configuration logic for both IOS and Android versions of Secure Email so its important to update the App Center App to match the version of the App Center so that it will work with the latest Secure Email version.
Please have them update their App Center App to match the App Center version, and do the following:
1. Remove the App center App from their device
2. On the IOS device, navigate to Settings->General->Profiles, and remove Remove the App Center profiles from their device (both email and MDM profiles)
3. Re-enroll the device
4. Install Secure Email, and start it. They should see a Symantec branded configuration screen which has some data pre-filled, based on their App Center Secure Email device policy.