The internal error is likely unrelated. Would need to know which portal you are logging into when you get that error.
You do not need proof of game ownership for your account to be able to log in and post on the forum. So you can log in with C_P and post here.
In order to log into the client, lobby server, and play the game you do need to have proof of game ownership. This is why only CP can login to the client. C_P cannot because it is not linked, it also cannot be linked to your steam account because each steam account can only be linked to one faf account.
You can log into the forum with CP and then you can post as that user. The accounts themselves share no data between them in our databases. It is probably only logging you in here as C_P because you have remember me set.