Hmm, I’ve not come across that issue before. My guess is that an “IPP” error is referring to iPhonePackager.exe which is the final step of build. Seems that it’s having an issue with the developer certificate.
What I do know from my troubles, is that when you import your developer certificate with the “Import Certificate” button in the iOS settings, what it actually does is adds it to your certificates in Windows (which I guess is the equivalent of Keychain access on mac).
Most likely your issue is to do with that process. If I run the program “certmgr” on Windows, I can see my certificate in Personal → Certificates. It has a small key on the icon, and under “Intended Purpose” is says “Code Signing”. Do you see that?
I think the first thing to try is reimporting the certificate in the Unreal iOS settings. It should ask you for the password you gave when exporting from Keychain Access on the Mac. (I’m assuming if you get the password wrong, it will tell you, but if it doesn’t, that could be the problem).
Out of interest, did you create a new 2nd Developer certificate on the Apple Developer site, or delete and re-create your original one?
I’ve also read that running Unreal in Administrator mode when you import the certificate can help, maybe it would even help for the build itself.
Other things to try…
Double check that your certificate and mobile provision are indeed the correct one in the iOS settings. I’ve noticed it sometimes seems to revert back to old ones.
You could try and delete your developer certificate in “certmgr” before trying to install it to unreal again. I’d recommend exporting it to a file before you delete anything, just in case…
Make sure you see a certificate called "Apple Worldwide Developer Certification " in certmgr. Open it and in the Details tab check the version, my one says V3. I believe that the developer certificate needs to have the “Apple Worldwide Developer Certification Authority” also installed in order to work. But this is normaly installed automaticaly as far as I can tell.
Hope this helps.