There would only be three reasons it can't open the file for writing.
1. Insufficient permissions. (Did you install the game into a folder underneath "C:\Program FIles" as opposed to your own user folder? If so, you may need to run the launcher as an administrator to update. I highly advise you to reinstall the game into a folder underneath your user folder so that both the game and launcher don't need admin permissions to do anything.)
2. Antivirus blocking creation of the ephinea.dll (Check your antivirus whitelist again and ensure the Ephinea folder is in there.)
3. Out of disk space. (I doubt this is the case, but you never know!)