Using Umbraco 6.1.6. 

I have one user that cannot get the backend to work in any browser (chrome, FF, IE) on one specific computer. If he goes to another computer or if I try to log in as him on my computer, everything seems to work fine. I've also tried creating a new user for him and he still experiences the same issue. No other users are experiencing issues. He has deleted his cookies and cache, no luck.

Specifically, the Content Tree loads and loads and eventually he will get the grayed out background, logged out screen. This was a copy of an older site, which we upgraded and then split in two separate sites where only CSS and Content nodes are different. The other site works fine for him, same server. 

Here's the error (note I replaced some info with Xs). Please let me know if I can provide any other information. Thank you!

Event code: 3005 
Event message: An unhandled exception has occurred. 
Event time: 9/29/2014 2:34:08 PM 
Event time (UTC): 9/29/2014 7:34:08 PM 
Event ID: 8ac3d12547d94220aeefb88d354faf3a 
Event sequence: 1265 
Event occurrence: 26 
Event detail code: 0 

Application information: 
    Application domain: /LM/W3SVC/63/ROOT-1-130564718682656250 
    Trust level: Full 
    Application Virtual Path: / 
    Application Path: XXXXXXXXXXXXXXXXXXXXXXXXXX
    Machine name: SFWEB02 

Process information: 
    Process ID: 4832 
    Process name: w3wp.exe 
    Account name: NT AUTHORITY\NETWORK SERVICE 

Exception information: 
    Exception type: InvalidOperationException 
    Exception message: The user has no umbraco contextid - try logging in
   at Umbraco.Web.Security.WebSecurity.ValidateCurrentUser(HttpContextBase httpContext, Boolean throwExceptions)
   at Umbraco.Web.Security.WebSecurity.AuthorizeRequest(HttpContextBase httpContext, Boolean throwExceptions)
   at umbraco.presentation.webservices.TreeDataService.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)



Request information: 
    Request URL: http://XXXXXXXXXXXXXXXXXXXXXXXXXX/umbraco/webservices/TreeDataService.ashx?rnd=44b2280d766d4ac8b5831312e61386ea&id=-1&treeType=content&contextMenu=true&isDialog=false&rnd2=98.7 
    Request path: /umbraco/webservices/TreeDataService.ashx 
    User host address: 150.70.97.121 
    User:  
    Is authenticated: False 
    Authentication Type:  
    Thread account name: NT AUTHORITY\NETWORK SERVICE 

Thread information: 
    Thread ID: 19 
    Thread account name: NT AUTHORITY\NETWORK SERVICE 
    Is impersonating: False 
    Stack trace:    at Umbraco.Web.Security.WebSecurity.ValidateCurrentUser(HttpContextBase httpContext, Boolean throwExceptions)
   at Umbraco.Web.Security.WebSecurity.AuthorizeRequest(HttpContextBase httpContext, Boolean throwExceptions)
   at umbraco.presentation.webservices.TreeDataService.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)


Custom event details: