Fortunately, we found the culprit for your issue, even though the error does not provide a meaningful message.
One of the iOS splash screens was with a wrong dimensions and this was causing the build failure.
In order to fix it, you will have to upload again your iOS iPhone Portrait (Retina)
splash screen (640x960) from Properties > iOS > Splash Screens window in our Windows client.
Normally, our clients automatically resize the pictures to the needed dimension, but this times most probably something has gone wrong.
I wonder whether you have used the same approach to change the custom splash screens or any other which has caused this misbehavior.
Which client did you use for the purpose?
I hope the fix will resolve your issue, but in case you still need further assistance please do not hesitate to write back.