user can't login after external login through OAuth2

I'm useing  asp.net 4.5.1 . I have migrated from Membership to ASP.NET Identity 2.0 .  I want  the external user  can be login my website throuht OAuth2 . And it can really work. Bute I meet a problem:  when the exernal users login my website . the inner user  can't login any more.  And ther are not error report.  After  I recovery  the  IIS Apppool, the inner user can login again. I don't know  why it can be this  how can over right ? 

 the follow is my external user login code 

.....
ViewState["OpenID"] = token.OpenId;
ViewState["AccessToken"] = token.AccessToken;
ViewState["RefreshToken"] = token.RefreshToken;
ViewState["NickName"] = qquerInfo.Nickname ;

.........
 var userManager = new UserManager();
 var user = userManager.FindByName(userName);
 IdentityHelper.SignIn(userManager, user, isPersistent: false);
 IdentityHelper.RedirectToReturnUrl(returnUrl, Response);
.................