Cannot login to ArcGIS Online from ArcGIS Desktop 10.5.1

[AllanAxon1]

I have a user who cannot login to AGOL from ArcGIS Desktop 10.5.1. Under Add Data, the Add Background later function is greyed out. Under the File Menu the Sign in option is greyed out. We have 100's of Desktop installations and only one user is having this problem. He is able to login and access our AGOL organization correctly when using any web browser. ArcGIS Desktop seems to be looking for a non-existent proxy and no proxy is configured in Arc or Windows.

Connection Test results:

Ran ArcGIS Connection Tests -
Proxy Server Test - Failed
Reason Unknown.

Our network does not use a proxy server. I've checked ArcCatalog network settings and Windows network settings to confirm no proxy information is incorrectly entered where there should be none. I have also had him replace Normal.mxt to see if it's an issue with any customizations he is currently using in his mxd files. The behavior is the same in multiple mxds.

Download Speed Test - Failed
Test site unreachable.

Network Latency Test - Passed
Network latency 0 ms. Rating Excellent

Network Route Test - Passed
Network route solved with 1 nodes. Route details :
XX.XX.XX.XX [<machine name here>]

[Robert_LeClair]

Allan - check to see if the ArcGIS Administrator on his manage has https://www.arcgis.com as the active portal.  Go to the ArcGIS folder by clicking the Windows start button bottom left, then ArcGIS-ArcGIS Administrator.  Click Advanced lower right on the first window, followed by ArcGIS Online->Manage Portal Connections.  In the Manage Portal for ArcGIS Connections, click https://www.arcgis.com and Connect.  Then Dismiss->Save->OK.  Restart ArcMap.  The user may have to log in under File->Sign in.

What is the result?

[AllanAxon1]

Robert,

Forgot to mention that we had done this. Here are the 2 articles we have been using to try to fix this issue:

https://support.esri.com/en/technical-article/000013176

https://my.usgs.gov/confluence/display/EGIS/Resetting+your+ArcGIS+application+profile

We have tried all the recommended steps in these 2 articles except completely resetting the user's profile in the 2nd url from the USGS. We are going to try this next, but I was hoping to avoid a drastic step like this. We have also uninstalled and then reinstalled ArcGIS Desktop.

[AllanAxon1]

I think that the issue is the failure to connect to the proxy server. Our users do not go through a proxy to access internet sites. ArcCatalog does not have a proxy server setup and Windows networking does not have a proxy set up in that configuration either. I have been trolling through user profile files to see if there is something hidden there.

Thanks, Allan

[Robert_LeClair]

I know this is true for ArcGIS Pro and "maybe" it would work here but renaming the Esri folder in the C:\users\<user_profile>\appdata\local and C:\users\<user_profile>\appdata\roaming to Esri_Old sometimes works too.  Somewhat of an extreme fix but works.

[AllanAxon1]

Tried creating a new user profile by renaming the ESRI folder but no change

[ShannonVeraldi2]

I have a user with this same issue - exactly the same. Any luck?

[cbollman]

I'm having this issue and it started out of nowhere! 

[BryceBeyers]

Were you able to find a fix for this? I had the same thing happen to me. No changes that I'm aware of and can't connect.