Token error when trying to see free/busy times in Outlook 2016

[Charlie_123]

Hi, we have a federation trust with another company and want to see the free/busy times in each others Outlook calenders.

From our four Exchange Servers it only works, if the user mailbox is located on a certain one, the other three are causing a token error when trying to access the calenders within Outlook 2016. The other company runs Office 365.

Federation Trust failed with Error:

Id         : TokenRequest

Type       : Error

Message    : Failed to request delegation token.

There don't seem to be any network issues. What could solve the issue? It has not worked before.

Thanks in advance!

[#2]

What does Test-FederationTrust -Verbose say?
Try Get-Federationtrust | Set-FederationTrust –RefreshMetadata to refresh the certificate a.o.

[#3]

Thanks, we have tried this, but it did unfortunately not solve the problem.

[#4]

Seems to me that some servers might not access the Internet.
Check Proxy Server Settings
Get-ExchangeServer | ft Name, *Proxy*
Netsh winhttp show Proxy

Regards
Andres