Hier steht, wie man sich genau bei dem Problem verhalten soll:
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What if I had a different email address on my forum account to my Steinberg ID?
If you’d like to bring everything across to your Steinberg ID, let us know your phpBB forum username, phpBB forum email address and your Steinberg ID and we’ll do the rest. If you’d prefer to keep your MySteinberg and forum accounts separate, then simply create a new separate Steinberg ID with your phpBB forum email address.