Marco,
I don't quite understand how you were able to resolve the issue.
I've logged out of my iCloud account, and logged back in to my iCloud account. I've moved the plist information from the preference folder to the desktop. I've disabled iMessage on my iPhone and re-enabled it. I've even checked the accounts on both my Mac and iPhone, and the email addresses match up identically. For whatever reason, I still cannot get my Mac to send or receive text message anymore. This started happening right when I updated to OS X El Capitan Beta 10.11.1. If you or anyone have any ideas on what to do, please do let me know.
Thank you.