@Berg, Ronald van den , Research for the error and find one link mentioned, it can be caused that the AD FS Windows Service is stopped on the federation server computer. We can verify that the AD FS Windows service is started on the remote federation server computer.
https://systemcenter.wiki/?GetElement=Microsoft.ActiveDirectoryFederationServices20.TokenIssuanceFederationServerMEXEndpointMonitor&Type=UnitMonitor&ManagementPack=Microsoft.ActiveDirectoryFederationServices.2.0&Version=7.0.8560.0
Note: Non-Microsoft link, just for the reference.
Hope it can help.
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