“Unable to validate account” at appleID sign in

You can reset the network setting of your IPhone (Go to Settings > General > Reset > Reset Network Settings > reboot your IPhone). Otherwise, you may try to turn off SSL to see if it works (go to Settings > Mail > Account > Exchange account > Account > Advanced > turn off use SSL).

I also had this problem and I realized the time and date on my phone were wrong. Once I updated the time and date correctly all verification worked. Make sure your location services are turned on and your time zone is set to automatic.

Hope this helps. Do let me know if it works. Thanks

Some things you can try. Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch         

Try a forced restart.

Reset Device.      

So glad it worked! :)