Sitefinity's sync feature requires a specific account to be created under which the operation runs. When doing SiteSync between 2 environments, the operation fails. The following error is being observed in the Source environment's Sync logs:

"ArgumentOutOfRangeException: Forbidden (403) is not one of the following: OK (200), Created (201), Accepted (202), NonAuthoritativeInformation (203), NoContent (204), ResetContent (205), PartialContent (206) ---> System.Web.HttpException: You are not authorized because the same user is logged in on the target server."

Checking the Target environment for the SiteSync account's status prior running the sync operation shows the user is not logged in. No one has logged with the account during the sync.
The SiteSync is working, however sometimes the account used for the sync will remain logged n on the target environment. When this happens all sync tasks fail until we manually log into the target server a force logout the sitesync account.

The problem has been encountered only in Sitefinity setup against Oracle database, where the Source environment is in NLB and the Target environment is a single server. The occurrence is intermittent. 

The exact cause is not known at this time. 

Solution for Sitefinity version 9.2 and below:
Find the attached CustomSitefinityClaimsAuthenticationModule.cs to this article.

1. Download and place in the root of your Target server application.

2. Open the web.config and replace the "SitefinityClaimsAuthenticationModule" with the "CustomSfClaimsAuthenticationModule".

- There should be 2 occurrences of it - 1 in the <system.webServer><modules> node and another in the <httpModules> node. They look something like this:
<add name="SitefinityClaimsAuthenticationModule" type="Telerik.Sitefinity.Security.Claims.SitefinityClaimsAuthenticationModule, Telerik.Sitefinity" />

- Replace the declaration with:
<add name="SitefinityClaimsAuthenticationModule" type="SitefinityWebApp.CustomSfClaimsAuthenticationModule, SitefinityWebApp" />

3. Open the CustomSitefinityClaimsAuthenticationModule file and modify the check for the Sync user name to the following:

if (user != null && user.UserName == "YourSyncUserName") //you need to modify this to your sync user name
{
SystemManager.RunWithElevatedPrivilege(DoWork, new object[] { user.ProviderName, user.Id }); 

}

4. Build and run the project

The call intercepts each request that tries to authenticate on the Target server. In case the request is from the SiteSync account, it logs it out before logging it back in, making sure the account is free to sync with.

Solution for Sitefinity 10.x and above versions:
1) From the source site for site sync subscribe to Sitefintiy site sync module SyncronizationEnd events (Sitefinity documentation,  site sync events# site sync completed event https://www.progress.com/documentation/sitefinity-cms/for-developers-site-sync-events#sitesyncmodule-synccompleted-event ) sample file attached.
Those events are triggered only at the source site that initiates site sync.
Those events are responsible to communicate to the destination site and check if the site sync user is logged in and if logged in then log it out.

2) The code on the destination site is to accept the ping from the source site, check if the site sync user is logged in and then log it out.

Refer to the attached file Global.asax.cs for the code.
What is not included is how to initiate a request to an endpoint on the destination server that is to log out the site sync user. We can sync on the best approach for this.
One way to do it is to have a REST web service that is the endpoint to logout a user. Sample here: Sitefinity documentation, create a restful wcf service to retrieve titles of news items https://www.progress.com/documentation/sitefinity-cms/72/tutorial-create-a-restful-wcf-service-to-retrieve-titles-of-news-items

Another way is to have a page with a widget that contains the logic for user logout.
A prerequisite is to have the site sync user logged out from the target environment and start site sync. The above solution will ensure that the site sync user always gets logged out when site sync completes.