[SOLVED → Reset NVRAM 3 times in a row] It might sound like a message for another forum, but hear me out. I have a problem on my 4,1→5,1 Mac Pro, where I get an "Activation Error" whenever I try to sign in to Messages. And it was working fine before, just stopped at some point (unfortunately I don't recall what preceded it). Now, I've researched the problem, and tried all possible solutions. Here are a couple of links summarizing most known solutions: http://tips.stevenchu.com/tech/mac-osx/solution-imessage-messages-on-mac-stopped-working-completely/ https://appletoolbox.com/error-occurred-during-activation-for-imessage-or-facetime-on-mac/ After I've tried everything, it came to this: most probably my Mac's serial number is blocked somewhere on iMessage servers due to too many incorrect logins or whatever. The only solution is to call Apple, and ask them to check for the block and remove it. And I've tried to do just that. I've called Apple Support and spend around an hour one the phone with them. When we got to the serial number check, I've realized it might not be as easy: My serial number belongs to 4,1, but I'm running Mojave, which is not supported on 4,1. I'm also spoofing the serial number to enable HEVC (as per this post) with the following line 'sudo nvram boot-args="shikigva=96 shiki-id=Mac-7BA5B2D9E42DDD94"', although it probably doesn't affect the real serial number, but I can't be sure. Apple support told me that they've tried to remove the block, but it didn't work out, because after another check it doesn't seem that I have a block on my Mac. After that they've told me the following: "Looks like it's not AppleID/iCloud issues, because it works on other Macs, so it's hardware issue. And because your computer is now obsolete, we cannot provide support for it. Goodbye." Question, did anyone here had a similar issue with Mac Pro? Is there any point trying to contact Apple again, but disabling all the spoofing beforehand? Maybe any other advice someone might give me?