Q: iMessage error during activation

I have a Mid 2011 Mac Mini, previously on Mavericks. Yesterday, I had to sign out of iMessage and log back in, but now I cannot sign back into two different iCloud accounts. I get the error "An error occurred during activation. Try again." I reformatted the computer (had been on my to do list) and thought It was some random glitch. I tried all of the suggestions on the website but no success! Thanks!!