We had a user with login "DOMAIN\user" he left the organization long back. We have a new user with same name, infra team created the same login name for this second user "DOMAIN\user". Now whenever the new user tries to login the site is not responding, I have to recycle the App pool or reset the IIS. I have SharePoint 2013, but the said portal is still in windows authentication.
That's an odd bug, but try running:
$user = Get-SPUser -Identity "DOMAIN\User" -Web https://webUrl Move-SPUser -Identity $user -NewAlias "Domain\User" -IgnoreSid
Make sure the first line,
Get-SPUser, points at a Site Collection where your previous user had access to (was added to the User Info List).
Once complete, try having your new user log in.