If you are using Apache as the reverse proxy for Lync, you may experience some issues when signing in using the mobile app (I only tested iOS). Things to look for are:

• 50/50 chance in sign-ins (first fails, second works, as shown below)
• Calls (audio or video) fail to establish
• Signing out of the application produces an error as shown below

To resolve, look for the DefaultType directive in your httpd.conf. If it’s set to text/plain, change it to None and reload Apache config. The line should read:

DefaultType None