While attempting a Lync conference with one of my customers we noticed the options for whiteboarding and PowerPoint were unavailable in the user interface. These items are served up by the Data MCU, and are not displayed if that MCU cannot be contacted for some reason. We took a look at the Lync server logs and found events like this one continuously appearing:
A Create Conference request sent to an Mcu was rejected. It will be retried but if this error continues to occur conferencing functionality will be affected.
Mcu: https://<Server FQDN>:444/liveserver/datamcu/ Conference: sip:<SIP URI>;gruu;opaque=app:conf:focus:id:FPGCC8M9 Error: otherFailure
Cause: Overloaded or incorrectly functioning MCU.
Resolution: Ensure that the Mcu is functioning correctly.
Everything else in the environment seemed ok, but I also noticed some errors around address book photo permissions going on so I took a peek at the back-end file share for web components. It appeared the file share NTFS permissions had somehow been reset to where the RTC groups did not have the necessary read and write access, so these errors were showing up. The fix was really easy: open Topology Builder, download the existing topology, and re-publish. Publishing the topology will verify the file share permissions and reset them if needed.