iCloud
when I try to login to iMessages with my Apple ID it says an unknown error occurred, but my password is correct
MacBook Air 13", macOS 10.15
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Messages - If you can‘t sign in to your iMessage account or FaceTime on your Mac
Have someone else attempt to sign in using their Apple ID. If they can successfully sign in and use Messages/FaceTime, it may be your Apple ID. Someone I know had this happen and I was able to sign in using my Apple ID. After a call to Apple Support and then being elevated to a tier 2 support person, it turned out it was a problem with their Apple ID. It took the software engineers 3-4 days to fix the problem. When calling Apple, make sure you tell them it is an Apple ID problem so you don't get charged.
You can also try signing in on another device/computer. There have been a couple of posts where people's device/computer was blocked. You will need to contact Apple for that problem also.
There is a contact link.
Apple Support by e-mail or chat
Or fill out and submit this form.