Symptoms
The ms-client-diagnostics header that's used to troubleshoot application sharing issue is not correctly formatted.
Cause
This issue occurs because the incorrect type is put in the MediaType filed in a diagnostic blob that is added to the ms-client-diagnostic.
Resolution
To fix this issue, install the May 2017 cumulative update 6.0.9319.281 for Skype for Business Server 2015, Conferencing Server.