Portal machine - Portal (using IWA) and WA 10.6.1 (WA has Windows Authentication only enabled)
WFM Server machine - ArcGIS Server and WA 10.6.1 (federated with Portal but not a hosting server) (WA has both Windows and Anonymous Auth enabled)
Please note that we do have the 10.6.1 hotfix to BUG-000115160 installed.
The issue can normally be resolved by a combination of restarting the WFM service or ArcGIS Server service.
We cannot understand why this keeps failing each day and requires a restart of the service. Any help would be greatly appreciated.