Hi, I have a Mac Pro Early 2009, running High Sierra (10.13.6). Roughly six days ago, I was kicked off my iMessage and since then I have been unable to sign back in. I'm 100% sure my password is good as I have used it since to purchase and to sign into iCloud. The error message I'm getting is "Could not sign in to iMessage." (An error occurred during activation. Try again). Also, the iMessage on my iPhone as well as my MacBook Pro (2011) works just fine. I've followed several online troubleshoot with no success. So finally I decided to call Apple support tonight, a very nice lady helped me with the entire process, asked a few questions and we tried a few things (nothing I have not tired already). Eventually she told me she would need to contact her senior engineer about this issue. After being on hold for roughly two minutes, she came back and told me because of the age of the Mac Pro 2009, Apple is no longer supporting iMessage on it. She said she felt that is a bit weird, but that is what the senior engineer told her to tell me. I'm not sure if I really buy this answer. I mean the iMessage was working until just six days ago. It stopped working all of sudden and now Apple is telling me my iMessage running High Sierra is no longer supported? Can anyone advise me on this? Thanks! Update: Solved, tl;dr, reset the NVRAM.