Allright, so I had that "Your AppleID can't be used to set up iMessage at this time" error and called Apple since all the other solutions I tried didn't work.
For people that are a bit afraid to call them don't worry, just be sure you have these things:
- A real Apple Hardware SerialNumber added to your bootloader, as many people already said, check Google and use:
https://selfsolve.apple.com to check it. Be sure the serial number matches your SystemProfile/SMBios e.g. iMac Late 2013 should have a late 2013 s/n.
- An iOS or other Mac with iMessage working (they will ask if you have, not needed but just in case..)
- The customer code that iMessage shows and iMessage opened at the login Window.
When you call them the following will happen.
- You hear an automatic voice telling you all the options, you want to connect to the computer helpdesk (I connected to "Other" and well, the call took 20 minutes with redirecting..).
- Tell them you have issues with logging in to iMessage on your iMac/Macbook/Mac Pro and that you're seeing a customer code (use the device from the real serial number)
- They will ask you for the code iMessage shows.
- They will ask for your AppleID that you're trying to log-in with.
- They will ask you if you have other devices with iMessage that works.
- After that they will probably put you on hold to enjoy some telephone quality music.
- Then they will ask you if you can login and it would work!
The guy from Apple didn't understand this error at all since it's very rare in Europe he said, I just thanked him and well that was it.
iMessage works again
This reminds me to never change my s/n again..