A recent case worked proved to be difficult to determine the cause. The issue shows up in the logs as Ms-Application-Aor header is not valid. While you can review the Blogs on the subject, the issue may be a lot simpler then it seems. After making changes to a large environment that has multiple pools and multiple SBA’s the answer was rather elementary: restart the Lync Conferencing Attendant and Lync Conferencing Announcement Services.
The problem is this error will likely take you down the path of UCMA issues. UCMA is not usually the problem. There could be other possibilities like the DC, or other server like the Lync servers themselves may need a restart. Please try these options before moving on to UCMA. You will likely find articles like this below:
Before you start manipulating the SIP headers manually (Lync Dev by Michael Greenlee), try restarting the conferencing attendant and Lync conferencing service. It will save you a headache. I found as a side note, if this issue is occurring on subsequent additions of offices (conferencing locations, Lync SBA etc…) with conferencing set up, these offices may also exhibit the issue until you finally restart said services. once this is done, this issue went away at all locations.