It's not a bug, it's a feature.
You have to continue other quests. At least the photoshooting with Kara comes first. Get to this point:
Especially Lena is important for Sofia. Get her always as far as you can. So you do Lena next and Kara if possible. If not, other quests first.
you must stop with Erica before this:
When you get that far, you will automatically ponder about John next time you sleep and then the conflict with Sofia arises. Meaning: Do not get Erica that far before finishing Sofia. You might have to do Erica quests though to continue other quests. Just stop with Erica before that hint.
This conflict might also be solved by future updates, when Ericas quest can be continued. In that case Erica is meant to be continued before Sofia.
Alas - these are only presumptions by me. It is the way, i finished the current version.
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