Thank you all for your suggestions - I've gotten a bit further
- OAuth is now working. Found a error in the WindowsAzureACS AuthServer configuration.
- The Invoke-RestMethod -Uri "https://autodiscover.domain.tld/autodiscover/autodiscover.json?Email=mymailbox@keyman .tld&Protocol=EWS&RedirectCount=5" -UserAgent Teams is working as well. Turned out to a client side issue. With only TLS1.2 enabled on the Exchange server I needed to force my powershell client to use TLS1.2 as well
Unfortunately the issue with the Teams client persists, and I've opened a case with Microsoft - I'll make sure to update this thread once it has been resolved. It may help others facing the same issue.
/mikkel