Domain administrator created an AD group with some users in it and we inserted it in SP group, but users don't have access. After three attempts they get access denied.
I've seen this behavior at one of our customers and we found that it took up to 24 hours before the AD-group was accessible in SharePoint. To overcome this, we created another SharePoint group, set the permissions on the group and added the effected users to the group.
The day after we checked with one effected user, removed her from the second group, and she had access then.
I don't know why, but as a workaround it's doable.