Q:

Facebook wont sign in on one specific account

We have recently implemented the Facebook SDK. For all our team and testers they can sign in with no issues; but for one team member it always gives a 'user canceled the login error' - it doesn't help that I'm on west coast and he is on the east coast so I can't debug it - however I do have the following print out of the error via TestFlight... 

2013-06-27 20:33:27 facebookFirstTimeLogInUserCanceled error:Error
  Domain=com.facebook.sdk Code=2 "The operation couldn’t be completed. 
(com.facebook.sdk error 2.)" UserInfo=0x1e5d3790
{com.facebook.sdk:ErrorLoginFailedReason=com.facebook.sdk:SystemLoginCancelled,
  com.facebook.sdk:ErrorInnerErrorKey=Error Domain=com.apple.accounts
  Code=7 "The Facebook server could not fulfill this access request:
  Service temporarily unavailable" UserInfo=0x1f0761c0
  {NSLocalizedDescription=The Facebook server could not fulfill this
  access request: Service temporarily unavailable},
  com.facebook.sdk:ErrorSessionKey=,
  expirationDate: (null), refreshDate: (null), attemptedRefreshDate:
  0001-12-30 00:00:00 +0000, permissions:(null)>}
2013-06-27 20:33:35 Error Domain=com.facebook.sdk Code=5 "The
  operation couldn’t be completed. (com.facebook.sdk error 5.)" 
UserInfo=0x1e5c2190  {com.facebook.sdk:ParsedJSONResponseKey={ body =
  { error = { code = 2500; message = "An active access token must be
  used to query information about the current user."; type =
  OAuthException; }; }; code = 400; },
  com.facebook.sdk:HTTPStatusCode=400} 2013-06-27 20:33:35 Unexpected
  error:Error Domain=com.facebook.sdk Code=5 "The operation couldn’t be
  completed. (com.facebook.sdk error 5.)" UserInfo=0x1e5c2190
  {com.facebook.sdk:ParsedJSONResponseKey={ body = { error = { code =
  2500; message = "An active access token must be used to query
  information about the current user."; type = OAuthException; }; };
  code = 400; }, com.facebook.sdk:HTTPStatusCode=400}
2013-06-27 20:33:44 TestFlight: End of Session

A:

We have had the exact some problem: Everybody was able to sign in – except for one user, who got the following error message when he wanted to log in via Facebook's Login dialog built into iOS 6:
The operation couldn’t be completed. (com.facebook.sdk error 2.)
FBErrorCategoryUserCancelled
When we couldn't figure out why the system login was cancelled (SystemLoginCancelled) and The Facebook server could not fulfill this access request, we decided to pass on iOS 6 native authentication and use fast-app-switching instead.
Bypassing iOS 6 native authentication can be done by using FBSession's (unfortunately deprecated) class method
+ (BOOL)openActiveSessionWithPermissions:(NSArray*)permissions
                            allowLoginUI:(BOOL)allowLoginUI
                       completionHandler:(FBSessionStateHandler)handler;

This method calls
+ (BOOL)openActiveSessionWithPermissions:(NSArray*)permissions
                            allowLoginUI:(BOOL)allowLoginUI
                      allowSystemAccount:(BOOL)allowSystemAccount
                                  isRead:(BOOL)isRead
                         defaultAudience:(FBSessionDefaultAudience)defaultAudience
                       completionHandler:(FBSessionStateHandler)handler;

with the third parameter (allowSystemAccount) set to NO, which causes the app to fall back on fast-app-switching.
When the user then tried to log in again, the following message appeared:

It turned out that the user had turned off platform applications, which prevented our app from integrating Facebook. After platform applications had been turned back on, the login was finally working.
In Facebook you can turn on platform applications by going to your Privacy Settings and selecting Apps from the main menu. Now you can click on Edit to reveal a button that says Turn On Platform.

