Lately I have start to see this type of error, either where EWS internal URL is blank also the External URL.
The Skype for Business is running on Windows server 2016, Exchange 2016 and Hybrid to O365. After lots of Google on this. Added Kerberos for Skype for Business. Changed OAuth certificate but the client got prompted about for Exchange credential all the time and it did not help at all give this to the client.
It was now time to ask my co-worker Ivar Naggum who is the Exchange Expert at blinQ about this, and together with Alexander Tuvström who was from the customer
We started to look for what is different here than for other customer on Exchange? We got information from the customer and Alexander Tuvström, they told that Exchange had been updated from 2010 to 2013 and then to 2016. Could it be some old stuff that is casing this trouble.
The customer represented by Alexander Tuvström logged on to Exchange and what we found was some old policies on Exchange that was the issue.
We have never seen this issue before and it was little of wild guess, but the expertise at the customer found this.
From this picture you can see what we did to fix this:
Hope this will help you or take you on the way to the solution. Thanks for the help from the customer (repesented by Alexander Tuvström).