In a One Identity Manager version 8.1.2 environment the API server was installed and the HTML applications (API Server UI and Operations Support Portal) were compiled successfully. Afterwards, access to the Operations Support Portal is fully functional. After 15 to 30 minutes, logging in is no longer possible.
When loading the login page the following error message appears:

From the API server log:
"ERROR ( WebLog ) : System.Exception: An error occurred while processing the request: https://domain.com/ApiServer/imx/sessions/opsupport ---> VI.Base.ViException: Error during execution of statement: select a.Ident_DialogAuthentifier, a.AssemblyName, a.Class, a.InitialData, a.Caption, db.IsMainDatabase
from DialogAuthentifier a with (nolock)
left outer join DialogProductHasAuthentifier pha with (nolock) on a.UID_DialogAuthentifier = pha.UID_DialogAuthentifier
left outer join QBMProduct p with (nolock) on pha.UID_DialogProduct = p.UID_DialogProduct
left outer join DialogDatabase db with (nolock)
on db.UID_DialogAuthentifier = a.UID_DialogAuthentifier
and db.IsMainDatabase = 1
where (a.IsEnabled = 1) and (a.ShowInInterface = 1) and (pha.IsInActive = 0) and (p.Ident_Product = N'OperationsSupportWebPortal')
order by a.SortOrder ---> VI.DB.DatabaseException: Database error 0: The CancellationTokenSource has been disposed. ---> System.ObjectDisposedException: The CancellationTokenSource has been disposed."