Account invalid, DLCs unauthorized, can't use Multiplayer (invalid account)

I've bought the Mass Effect Trilogy for PC (on Discs, not only a product key) and I have downloaded and installed Mass Effect, Mass Effect 2 and Mass Effect 3 on Origin. After finishing Mass Effect, I imported my ME1 character into Mass Effect 2. After finishing ME2, I imported that character into Mass Effect 3. Aditionally, I downloaded all of the free DLCs for Mass Effect 3 (Earth, Rebellion, Extended Cut, Retaliation, Reckoning and the Resurgence Pack).

I started Mass Effect 3 and there was an error message saying my account was invalid. After pressing "OK", the game tried to connect to the EA-Servers, only for another error message to show up, saying all the DLCs I've already downloaded were "not authorized", suggesting I'd have to log in with the Origin account I've purchased these DLCs with. I only have ONE origin account, so having used another account to download these DLCs can't be the cause of this problem. After those two error mesages I now can't use any of the contents I have downloaded, also i can't connect to the multiplayer.

I already tried repairing and updating the main game (Mass Effect 3) and all of the DLCs I have downloaded, but this didn't seem to work. I still can't use the multiplayer option nor the DLCs.I hope you can help me

I've tried contacting EA Support via E-Mail, but the case was closed without offering me a solution. Also, I can't seem to resume that case via E-Mail, why is that?

Thanks a lot