Changed Apple ID password, now "An unknown error occured." when trying to sign into iCloud on OS X

OS X El Capitan Version 10.11 · MBP 7,1 · 2.4 GHz Core 2 Duo · 8 GB 1067 MHz DDR3

I had to change my iTunes store on my iPhone to another country to download an app and it required me to change my password, which infuriated me because I had just changed within the past month or two (which is a whole other issue). But now I've been getting prompts on my Mac that it "can't connect to iCloud because of a problem with" my Apple ID. So I click on "iCloud Preferences..."

where it takes me to "Sign in to use your Apple ID." I oblige. But then I get an error back saying "An unknown error occurred."

And can anyone tell me why the only file type for Grab that you can save screenshots in is TIFF when Apple Support Communities only accepts JPG, GIF, or PNG? It sure would make things easier if you could just save as a JPG by default.

Any help would be appreciated. The constant popping up of the "This Mac can't connect..." is quite annoying. What's more is that I hardly use iCloud.