Index Out of Range Exception - Active Directory Adapter - asp.net-mvc

Question

I understand what an Index Out of Range Exception is, however, I can't seem to figure out what is going on in the Stack Trace below.
This is an asp.net website, and it has been working without issue for the past few years. I have checked with the System Administrator and they noted nothing has changed on the server hosting the website or dc. Currently at a stand still, any ideas as to where to look or begin diagnosing?
[IndexOutOfRangeException: Index was outside the bounds of the array.]
System.DirectoryServices.AccountManagement.SidList.TranslateSids(String target, IntPtr[] pSids) +1507
System.DirectoryServices.AccountManagement.SidList..ctor(SID_AND_ATTR[] sidAndAttr) +142
System.DirectoryServices.AccountManagement.ADStoreCtx.GetGroupsMemberOfAZ(Principal p) +220
System.DirectoryServices.AccountManagement.UserPrincipal.GetAuthorizationGroupsHelper() +50
PayDirectory.Classes.ActiveDirectoryAdapter.IsUserAllowed(String username, IList`1 allowedgroups) in C:\Users\johndoe\Documents\Visual Studio 2010\Projects\PayDirectory\PayDirectory\Classes\ActiveDirectoryAdapter.cs:35
PayDirectory.Controllers.AccountController.CheckUser(String sessionid) in C:\Users\johndoe\Documents\Visual Studio 2010\Projects\PayDirectory\PayDirectory\Controllers\AccountController.cs:42
lambda_method(Closure , ControllerBase , Object[] ) +108
System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +17
System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +188
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +27
System.Web.Mvc.Async.<>c__DisplayClass42.<BeginInvokeSynchronousActionMethod>b__41() +28
System.Web.Mvc.Async.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _) +12
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +50
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +29
Etc.
That is the beginning of the Stack Trace, I can post the entire trace if necessary.
Basically, the application takes the session id of a user logged into Joomla, checks ad to see if that user is in a particular group (Principals) and allows them to view certain content, otherwise it will state Access is Denied.
EDIT
Adding source code for "IsuserAllowed" in ActiveDirectoryAdapter class
public Boolean IsUserAllowed(string username, IList<string> allowedgroups)
{
PrincipalContext domain = new PrincipalContext(ContextType.Domain, "Redmond");
UserPrincipal user = UserPrincipal.FindByIdentity(domain, username);
List<GroupPrincipal> groups = new List<GroupPrincipal>();
if (user != null)
{
PrincipalSearchResult<Principal> authGroups = user.GetAuthorizationGroups();
foreach (Principal p in authGroups)
{
if (p is GroupPrincipal)
{
groups.Add((GroupPrincipal)p);
}
}
}
string groupstring = string.Join(",", (object[])groups.ToArray());
string[] groupnames = groupstring.Split(',');
for (int i = 0; i < allowedgroups.Count; i++)
{
if (groupnames.Contains(allowedgroups[i]))
{
return true;
}
}
return false;
}